Questions tagged [copy-on-write]

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Is copy-on-write not implemented based on page fault?

Operating System Concepts say fork() we can use a technique known as copy-on-write, which works by allowing the parent and child processes initially to share the same pages. ... When it is ...
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1answer
439 views

fork() and COW behavior after exec()

We understand the COW behavior after a fork (as for example described here) as follows: fork creates a copy of the parent's page table for the child and marks the physical pages read only, so if any ...
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0answers
167 views

can two running processes share the complete process image in physical memory, not just part of it?

can two running processes share the complete process image in physical memory, not just part of it? Here I am talking about the Linux operating systems(eg. Ubuntu). My thinking: I think it is ...
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1answer
32 views

Report directories with contents that exist elsewhere even if scattered

I want to generate a report of directories that I know I can safely delete (even if requiring a quick manual verification), because I know that the full contents all the way down, exist elsewhere--...
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1answer
210 views

Doesn't the existence of LVM snapshots slows down writing a file system wich doesn't support them natively?

As far as understand snapshots in LVM (please, do correct me if I'm wrong): since they are not persistent and work even with a file system which doesn't itself support snapshots => I suppose it must ...
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1answer
256 views

Will writing identical data to blocks of a file under ZFS use space in snapshots?

I have a 16M file. I take a snapshot of the ZFS filesystem which contains it. If I overwrite the file with the same data, will ZFS need to store two copies of all of the blocks of the file?
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2answers
4k views

btrfs — Is it dangerous to defragment subvolume which has readonly snapshots?

If you open the defragment section of btrfs-filesystem(8), you will see the following ominous inscription left by the developers: Warning: Defragmenting with Linux kernel versions < 3.9 or ≥ 3....
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4answers
6k views

Does any file system implement Copy on Write Mechanism for CP

We have seen OS doing Copy on Write optimisation when forking a process. Reason being that most of the times fork is proceeded by exec , so we don't want to incur the cost of page allocations and ...
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1answer
452 views

copy on write and read only files - linux

Suppose I have a single process which has some read-only pages, and it tries to write to one of those pages. This of course will raise a read-fault. My question is: does linux knows that the read-...
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2answers
1k views

BTRFS inside a KVM-VM on a qcow2 formatted image

I have a Ubuntu 14.04 application appliance that makes heavy use of BTRFS snapshots. The application is meant to be hypervisor agnostic and the snapshots need to be stored with the virtual machine ...
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1answer
328 views

How is btrfs (copy-on-write) different than RAID?

Whenever I hear copy-on-write my brain immediately starts thinking RAID. How is COW any different from RAID-x version ? Both need more than 1 HDD to be useful, both make a copy of data when data is ...
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1answer
123 views

How can I tell which copies of a file are COW copies?

I have a ZFS system connected to a macintosh over NFS. The system is nearly full, but I have found a large number of duplicate files with jdupes. I don't know how these copies were made, it's ...