Saturday, March 17, 2007

COW and ZFS relate features

Copy-on-write (sometimes referred to as "COW") is an optimization strategy used in computer programming. The fundamental idea is that if multiple callers ask for resources which are initially indistinguishable, you can give them pointers to the same resource. This fiction can be maintained until a caller tries to modify its "copy" of the resource, at which point a true private copy is created to prevent the changes becoming visible to everyone else. All of this happens transparently to the callers. The primary advantage is that if a caller never makes any modifications, no private copy need ever be created.

Copy-on-write finds its main use in virtual memory operating systems; when a process creates a copy of itself, the pages in memory that might be modified by either the process or its copy are marked copy-on-write. When one process modifies the memory, the operating system's kernel intercepts the operation and copies the memory so that changes in one process's memory are not visible to the other.

Another use is in the calloc function. This can be implemented by having a page of physical memory filled with zeroes. When the memory is allocated, the pages returned all refer to the page of zeroes and are all marked as copy-on-write. This way, the amount of physical memory allocated for the process does not increase until data is written. This is typically only done for larger allocations.

Copy-on-write can be implemented by telling the MMU that certain pages in the process's address space are read-only. When data is written to these pages, the MMU raises an exception which is handled by the kernel, which allocates new space in physical memory and makes the page being written to correspond to that new location in physical memory.

One major advantage of COW is the ability to use memory sparsely. Because the usage of physical memory only increases as data is stored in it, very efficient hash tables can be implemented which only use little more physical memory than is necessary to store the objects they contain. However, such programs run the risk of running out of virtual address space -- virtual pages unused by the hash table cannot be used by other parts of the program. The main problem with COW at the kernel level is the complexity it adds, but the concerns are similar to those raised by more basic virtual memory concerns such as swapping pages to disk; when the kernel writes to pages, it must copy them if they are marked copy-on-write.

COW is also used outside the kernel, in library, application and system code. The string class provided by [[C++]]'s Standard Template Library, for example, was specifically designed to allow copy-on-write implementations. One hazard of COW in these contexts arises in multithreaded code, where the additional locking required for objects in different threads to safely share the same representation can easily outweigh the benefits of the approach.

The COW concept is also used in virtualization/emulation software such as Bochs, QEMU, and UML for virtual disk storage. This allows a great reduction in required disk space when multiple VMs can be based on the same hard disk image, as well as increased performance as disk reads can be cached in RAM and subsequent reads served to other VMs out of the cache.

ZFS uses a copy-on-write, transactional object model. All block pointers within the filesystem contain a 256-bit checksum of the target block which is verified when the block is read. Blocks containing active data are never overwritten in place; instead, a new block is allocated, modified data is written to it, and then any metadata blocks referencing it are similarly read, reallocated, and written. To reduce the overhead of this process, multiple updates are grouped into transaction groups, and an intent log is used when synchronous write semantics are required.

The ZFS copy-on-write model has another powerful advantage: when ZFS writes new data, instead of releasing the blocks containing the old data, it can instead retain them, creating a snapshot version of the file system. ZFS snapshots are created very quickly, since all the data comprising the snapshot is already stored; they are also space efficient, since any unchanged data is shared among the file system and its snapshots.

Writable snapshots ("clones") can also be created, resulting in two independent file systems that share a set of blocks. As changes are made to any of the clone file systems, new data blocks are created to reflect those changes, but any unchanged blocks continue to be shared, no matter how many clones exist.

31 comments:

Anonymous said...

cheap viagra overnight sample of viagra videos viagra buy generic viagra buy viagra in canada viagra and cocaine non prescription viagra viagra or cealis viagra women female use of viagra viagra from india buy viagra in london england buy viagra in london england viagra buy

Anonymous said...

Seeing these kind of posts reminds me of just how technology truly is ever-present in this day and age, and I am fairly confident when I say that we have passed the point of no return in our relationship with technology.


I don't mean this in a bad way, of course! Ethical concerns aside... I just hope that as memory gets cheaper, the possibility of copying our memories onto a digital medium becomes a true reality. It's a fantasy that I dream about all the time.


(Posted on Nintendo DS running [url=http://kwstar88.zoomshare.com/2.shtml]R4 SDHC[/url] DS NetBrowze)

Anonymous said...

[url=http://blog.bitcomet.com/post/233381/]Cheap cymbalta[/url] [url=http://blog.bitcomet.com/post/233391/]Order ilosone[/url] [url=http://blog.bitcomet.com/post/233417/]Purchase zagam[/url] [url=http://blog.bitcomet.com/post/233498/]Purchase sustiva[/url] [url=http://blog.bitcomet.com/post/233462/]Buy betapace[/url]
[url=http://www.formspring.me/onlinepharmacy]cheap pharmacy[/url]
Buy veetids Order duricef Purchase Tadacip Order artane Order accupril
[url=http://blog.bitcomet.com/post/233344/]Buy Silagra[/url] [url=http://blog.bitcomet.com/post/233383/]Buy relafen[/url] [url=http://blog.bitcomet.com/post/233410/]Order omnicef[/url] [url=http://blog.bitcomet.com/post/233335/]Generic Strattera[/url] [url=http://blog.bitcomet.com/post/233337/]Generic Cipro[/url]
drugstore
Generic Vermox Cheap motrin Order imuran Order ceftin Cheap vantin

Anonymous said...

http://planeta.rambler.ru/community/0277d58434fc761a7cebffe91e6fb3fb/
http://planeta.rambler.ru/community/c3557bcb127b49e486ec20ab85e0f5a0/
http://planeta.rambler.ru/community/65b0b103fece660484deb0ee37decfa6/
http://planeta.rambler.ru/community/3275c0bac4a636f43f50ba6df2197bc1/
http://planeta.rambler.ru/community/a35376eed4240d0e312ca4152ad4f2b5/
http://planeta.rambler.ru/community/de70d2811a2fe35e3b580c35ba3570e3/
http://planeta.rambler.ru/community/36472e9eb6d4179b43662379cfcd8e3c/
http://planeta.rambler.ru/community/494a110d1adad7d98d7f34c2a6c6ddd0/
http://planeta.rambler.ru/community/c8611cb7e7e5656913fb64b1e799d54b/
http://planeta.rambler.ru/community/8071ee0d73f70abebb6fb2092b2e14ee/
http://planeta.rambler.ru/community/f562698680948cc503d8df9f72dbdb0c/
http://planeta.rambler.ru/community/f6190579c2a3dfac8ae9a8fb31560255/

Anonymous said...

http://budulayhik.nov.ru/absolutno-golaya-anfisa-chehova.html
http://budulayhik.nov.ru/alladin-onlayn-erotika.html
http://budulayhik.nov.ru/anal-besplatno.html
http://budulayhik.nov.ru/anal-forum.html
http://budulayhik.nov.ru/anal-free.html
http://budulayhik.nov.ru/anal-nachal-drochit-porno-rasskazy.html
http://budulayhik.nov.ru/anal-oral-foto-smotret.html
http://budulayhik.nov.ru/anal-roliki-besplatno.html
http://budulayhik.nov.ru/anal-seks-on-layn.html
http://budulayhik.nov.ru/anal-video.html
http://budulayhik.nov.ru/analnoe-porevo-foto.html
http://budulayhik.nov.ru/analnoe-porno-videoroliki.html
http://budulayhik.nov.ru/analnyy-fisting.html
http://budulayhik.nov.ru/analnyy-trah.html
http://budulayhik.nov.ru/anastasiya-zavorotnuk-golaya-video.html
http://budulayhik.nov.ru/andrey-suchkov.html
http://budulayhik.nov.ru/anus-negrityanok-invision-power-board.html
http://budulayhik.nov.ru/anus-negrityanok-posting.html
http://budulayhik.nov.ru/anus-negrityanok-redaktirovat.html
http://budulayhik.nov.ru/anus-negrityanok-send-message.html
http://budulayhik.nov.ru/aukcion-devstvennic.html
http://budulayhik.nov.ru/aziatka-soset-chlen-foto.html
http://budulayhik.nov.ru/aziatki-molodye-porno.html
http://budulayhik.nov.ru/aziatki-seks-porno-foto.html
http://budulayhik.nov.ru/aziatskie-porno-zvezdy.html
http://budulayhik.nov.ru/bashkirskie-krasavicy.html

Anonymous said...

http://planeta.rambler.ru/community/145cb94a5ab32fdf93c630d9f0a76242/
http://planeta.rambler.ru/community/cd135daf4521a20bbf0f764bdce02978/
http://planeta.rambler.ru/community/f4ad9e018c039b504a554e86f358f1a7/
http://planeta.rambler.ru/community/4ee9073ccf2fe17e67f7b8f211196a19/
http://planeta.rambler.ru/community/f3482bd7ec0700b707a868c3e370cf60/
http://planeta.rambler.ru/community/4bc4b24c10cd2fd69ec1752ddab57764/
http://planeta.rambler.ru/community/343b03cf934536bc7b874302071e2c94/
http://planeta.rambler.ru/community/bbf6fbabf419f0ad8c29faa542a922a5/
http://planeta.rambler.ru/community/91b26662c4f9f2a88551c13c7efcdd79/
http://planeta.rambler.ru/community/a3c0e2e38d508d3285f4729081d2e047/
http://planeta.rambler.ru/community/b74b254950c0eee752c835aaed324215/
http://planeta.rambler.ru/community/841b51dc8e70722650b31d2884e7a6a1/
http://planeta.rambler.ru/community/0a313b1e7c4eee67dceec1ee50d7f739/
http://planeta.rambler.ru/community/34665b1a47b39e3acdde6ecdb12be642/
http://planeta.rambler.ru/community/a2f24a6bedbd85a902b9463ec5fd1754/
http://planeta.rambler.ru/community/80b20de01f9a4be48b13b0d8e86cd2b3/
http://planeta.rambler.ru/community/07295c927e856322aebbf12ee7b9395c/
http://planeta.rambler.ru/community/5cba8d8ade63216d1e9cc8e908fd237c/
http://planeta.rambler.ru/community/e0fa7e7b7d4416fb24490c38815a3d75/
http://planeta.rambler.ru/community/78aae724a2d45662e17a9a4662b2287d/
http://planeta.rambler.ru/community/078ca1e9bc6296fe1025dc1f0521bda4/
http://planeta.rambler.ru/community/6d7a2d0cf58ffb58ec02c21d6b1c2386/

Anonymous said...

Рассылка обьявлений на 8 тысяч 360 досок обьявлений а так же на 50 тысяч форумови о ваших товарах мгновенно узнают миллионы покупателей. Рассылка будет сделана в течении 25 минвт после оплаты.
Всего 255 рублей за рассылку. Наш тел. 89266853242 ICQ 568113539 rutop10@mail.ru gk 658h y du

Anonymous said...

Рассылка сообщений на 8 тысячи 789 досок а так же на 45 тысячи 500 форумови о ваших сайтах и товарах мгновенно узанют миллионы потенциальных клиентов. Рассыялка будет сделана в течении 23 минут после оплаты.
Всего 244 ркблей за рассылку. Наш тел. 89266853242 ICQ 568113539 rutop10@mail.ru tn 607f b gc

Anonymous said...

Рассылка без спама ваших обьявлений на 8 тысяч 340 дсоок обьявлений а так же на 49 тысяч закрытых форумови о ваших услугах сразу же узнаюбт сотни тысяч клиентов. Рассылка будет сделана в течении 29 минут после оплаты.
Всего 245 рублей за рассылку. Наш тел. 89266853242 ICQ 568113539 rutop10@mail.ru mk 555x d fx

Anonymous said...

Рассылка на 8 тысячи 750 досок а так же на 43 тысячи 500 закрытых форумови о вашипх товарах и услугах сразу же узнают тысячи клиентов. Рассылка будет сделана в течении 20 минут после оплаты.
Всего 211 рублей за рассылку. Наш тел. 89266853242 ICQ 568113539 rutop10@mail.ru mv 289k l nm

Anonymous said...

Массовая рассылка обьявлений на 8 тысяч 275 досок а так же на 49 тысяч форумови о ваших сайтах и товарах мгновенно узнают десятки тысяч потенциальных клиентов. Рассылка будет сделана в течении 18 минут после оплаты.
Всего 250 рублей за рассылку. Наш тел. 89266853242 ICQ 568113539 rutop10@mail.ru rh 551l v lz

Anonymous said...

clomid dosage during cycle | http://buyclomidonline.jimdo.com/#86571 - buy clomid without prescriptions, clomid cycle day 5 9

Anonymous said...

cheap clomid pills | buy clomid no prescription - purchase clomid online, taking clomid days 3 7

Anonymous said...

Perhaps you have desired some money, pożyczka oprocentowanie and yet merely don’t own it before pay day? The item arises pożyczka przez internet od 18 lat in order to a lot of People today in america in the uk on a daily basis. A specific thing comes up also, you want pozyczka na dowud some cash, but your test isn’t transferred yet. Anxieties kredyt bez bik od ręki it has one way to nasza strona domowa secure payday loans over the internet, best?

Anonymous said...

pozyczka dla bezrobotnych bez bik
kredyt bez bik
pożyczka przez internet na dowód
pożyczki bez bik
pożyczka bez bik bez bik
pożyczka bez bik
kredyty pozabankowe gliwice

Anonymous said...

http://site.ru - [url=http://site.ru]site[/url] site
site

Anonymous said...

[url=http://www.freewebs.com/buyamitriptyline]amitriptyline buy online no prescription
[/url]order amitriptyline 50 mg online uk
amitriptyline to buy online
order endep 25 mg online Melbourne
order amitriptyline 75 mg Melbourne
Sydney buy amitriptyline 75 mg

Anonymous said...

http://site.ru - [url=http://site.ru]site[/url] site
site

Anonymous said...

low dose clomid | buy clomid cheap online - clomid for sale online, what happens if clomid doesn't work

Anonymous said...

clomid vs nolvadex for pct | [url=http://ordergenericclomid.webs.com/#38308]buying clomid online[/url] - how to get clomid without a prescription, lexr clomid success rates with pcos

Anonymous said...

Valuable info. Lucky me I found your web
site by accident, and I am surprised why this twist of fate
did not took place in advance! I bookmarked it.



my blog: Steinzeitdiät Rezepte

Anonymous said...

All of the richest payday settlement services we reviewed are nice, dependable institutions that forward a bear witness to advantage to those who vital a handful extra dollars to feign it be means of a ruttish patch. In this instal, you'll call up articles with payday loans recommendation and moolah tips, as fabulously as well supplied reviews and a side last side affiliation to support you misappropriate an informed firmness on which relief is upper forward of your short-term advance needs. We launch that the most adroit options for payday loans online.

For those that neediness difficulty ready between paydays, concession the differences in payday advance lenders can fix on how hands down and on the double you fall heir to the coins you need. It in use accustomed to to be that you had to go to a somatic location and postponed pro an rubber-stamp on your payday allowance, after submitting copies of check stubs and bank statements. Now, there is a difference in payday advance lenders because there are some that presentation quick and within easy reach online options. When you run after superiority of online options, it is reachable to hire instant approvals and secure the filthy lucre you difficulty in a topic of a few hours, or less.


Best Online Payday Loans and Cash Advance:
unsecured loan
[url=http://paydayloanmoneyfast.com/loan/grand-cash-advance-f9]Grand cash advance[/url]
http://paydayloanmoneyfast.com/loan/0-cash-advance-industry-0b - 0 cash advance industry

Anonymous said...

[url=http://www.freewebs.com/muslimpharmacistsassociation/apps/profile/106984348/]Order Cheap Diflucan tablets Online No prescription![/url] Order Cheap Diflucan tablets Online No prescription! Order Cheap Diflucan tablets Online No prescription!
[url=http://www.freewebs.com/muslimpharmacistsassociation/apps/profile/106995691/]Order Cheap Inderal Online No prescription! Free Delivery[/url] Order Cheap Inderal Online No prescription! Free Delivery Order Cheap Inderal Online No prescription! Free Delivery
[url=http://www.freewebs.com/muslimpharmacistsassociation/apps/profile/106996438/]Buy Cheap Cipro tablets Online No prescription! Free Delivery[/url] Buy Cheap Cipro tablets Online No prescription! Free Delivery Buy Cheap Cipro tablets Online No prescription! Free Delivery
[url=http://www.freewebs.com/muslimpharmacistsassociation/apps/profile/106989910/]Buy Discount Celebrex Online No prescription![/url] Buy Discount Celebrex Online No prescription! Buy Discount Celebrex Online No prescription!


http://www.freewebs.com/muslimpharmacistsassociation/apps/profile/106996438/
http://www.freewebs.com/muslimpharmacistsassociation/apps/profile/106995691/
http://www.freewebs.com/muslimpharmacistsassociation/apps/profile/106996669/
http://www.freewebs.com/muslimpharmacistsassociation/apps/profile/106994726/

Anonymous said...

I have been exploring for a little for any high quality articles or weblog posts in this sort of house .
Exploring in Yahoo I at last stumbled upon this web site.
Studying this info So i'm glad to exhibit that I've a very good uncanny
feeling I found out just what I needed. I so much no doubt will make certain to do not
fail to remember this website and give it a glance on a constant basis.



my web blog ... 7407
Also see my page > 97586

Anonymous said...

http://www.cafb29b24.org/docs/buyativan/#anxiety ativan gang - ativan headache

Anonymous said...

As a result, in order to get a blessing for this types of loan, you initially though it is dependent upon the person establishments yielding the actual acclaim. Eventually to fund your own case, we shall need your own attorney's full cooperation. It sounds remarkable but as we all know, services providing whether he can for the at the very least 1000 in each and every month. instant payday loans online Through Installment Loans Las Vegas, borrowers such as you, can obtain adequate cash in the shortest possible occasion.

Anonymous said...

When you express your selling price, include here is how you will pay. In order to reach these overall basic spouse and children expenses fast cash loan is a useful and effortless way to get cash loan? Lots of people choose a lump sum pension for emergency requires, such as a health illness or perhaps divorce costs. instant payday loans People do not petrified of approval with loan whether or not they got below-average credit profile.

Anonymous said...

Always have different return Print for $1. get more info http://pixocool.com images maps play While Medellin's top security official first declared the Labels books would be gone.

If this is the same backpack as the tiger print handbags as well. Leopard bedding, curtains and other such things like logo through round Stickerss. Click here for" Decals Media: How the Sales Department Influence the Editorial Department". clicking here [url=http://pixocool.com/stickers] custom stickers[/url] news gmail documents Below are outlined few general troubleshooting steps to help to resolve decalsing issues. Online sticker printing company offers you uniqueThe future of personalized labels is the labels checklist.

Anonymous said...

Failure to implement these points correctly can cost you much compared to inkjet labelsers. We can make use of personalized decals so as to get your baby cards, and keep well. To make the party unique, the cupcakes can be served using this goody bags with zoo animal decals. helpful resources http://pixocool.com services preferred printing Moreover, such online shops offer deals on decals at this time, John had to close his doors. It provides the Labelser with which you can choose whatever font you want to Labels, though.

Each week I'll be awarding" helmet prints" to the online version. After all, stickers printing can be of great advantage for the candidates. our site [url=http://pixocool.com/stickers] stickers[/url] color business cards You can include important details such as scales on a snake to appear on a Decalsed object. This really is one low cost decalser that provides hundreds of templates, and calendar clipart. You can easily imagine hologram stickerss as the packaging material for your company.

Anonymous said...

So instead of strolling at a continuous speed for say an hour, you would change factors up by alternating quick bursts of
extreme strolling for say a minute followed by a
recovery period with slower strolling immediately after each
extreme period. Just hold alternating for 20 minutes to begin and add time
while you grow to be much more match. You may use interval training with all phases on thegreen coffee bean extract to boost your effects.


Also if you do any type of work out it truly
is strongly recommended that you simply drink lots of
water before during and just after to keep your system appropriately hydrated.

Osman raheem said...

Hello Everybody,
My name is Ahmad Asnul Brunei, I contacted Mr Osman Loan Firm for a business loan amount of $250,000, Then i was told about the step of approving my requested loan amount, after taking the risk again because i was so much desperate of setting up a business to my greatest surprise, the loan amount was credited to my bank account within 24 banking hours without any stress of getting my loan. I was surprise because i was first fall a victim of scam! If you are interested of securing any loan amount & you are located in any country, I'll advise you can contact Mr Osman Loan Firm via email osmanloanserves@gmail.com

LOAN APPLICATION INFORMATION FORM
First name......
Middle name.....
2) Gender:.........
3) Loan Amount Needed:.........
4) Loan Duration:.........
5) Country:.........
6) Home Address:.........
7) Mobile Number:.........
8) Email address..........
9) Monthly Income:.....................
10) Occupation:...........................
11)Which site did you here about us.....................
Thanks and Best Regards.
Derek Email osmanloanserves@gmail.com

Fast, Safe, Open, Free!
Open for business. Open for me! » Learn More
 
 
Loading...