btrfs defra
Hello everybody,
I'm no developer, just a simple user. I use btrfs as the root filesystem on my laptop. I use VirtualBox. Every VB disk lives in a separat btrfs subvolume. I use btrfs snapshots for backup and to clone VB VMs.
I wonder what happens when I defrag a file that is part of multiple snapshots? Will all instances of this file be defragmented, or will the file get its own fresh copy?
If it is important, my system is a 64bit ubuntu 10.10 and the filesystem is mounted with the option compress. If it helps, I would consider to upgrade to 11.04.
BTW: what is the difference between defrag and balance ?
Sincerely
Bernhard
Quoted 7 lines · Bernhard Duebi
I'm no developer, just a simple user. I use btrfs as the root filesystem on my laptop. I use VirtualBox. Every VB disk lives in a separat btrfs subvolume. I use btrfs snapshots for backup and to clone VB VMs.
I wonder what happens when I defrag a file that is part of multiple snapshots? Will all instances of this file be defragmented, or will the file get its own fresh copy?
The file gets its own copy.
Quoted 5 lines
If it is important, my system is a 64bit ubuntu 10.10 and the filesystem is mounted with the option compress. If it helps, I would consider to upgrade to 11.04.
BTW: what is the difference between defrag and balance ?
Defrag works on individual files, and tries to find a contiguous
sequence of bytes to write the file's data to. In the process, the current implementation will break any CoW duplication -- either within single files (copies with cp --reflink=always) or files copied via snapshotting.
Balance works at a much higher level, rewriting all (or large
quantities) of the filesystem data and metadata. Balance was originally designed to re-allocate all of the space the FS needs, thus enabling you to add more disks to a filesystem and ensure that they get used. (It's also useful for re-building spare copies of data if a disk's been removed as a result of failure). Balance will maintain all existing file semantics as it runs -- CoW copies are preserved, and no additional space will be used.
Hugo.
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--- Someone's been throwing dead sheep down my Fun Well --- Quoted 1 line
=C2=A0 Defrag works on individual files, and tries to find a contiguo=
us
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sequence of bytes to write the file's data to. In the process, the current implementation will break any CoW duplication -- either withi=
n
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single files (copies with cp --reflink=3Dalways) or files copied via snapshotting.
While it's obviously impossible to defrag and maintain deduplicated partial chunks simultaneously without some kind of compromise (the ideal form of which is just beyond the reach of this list participant's obviousness flashlight), how difficult is it going to be to modify the current defrag implementation to hip the additional references to the file to the fact that a contiguous version (of an individual file) is now available and it's time to update their links too, after each file with multiple reflinks/snapshots to its current form has been copied to contiguous space?
Could Bernhard Duebi do it?
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Quoted 17 lines · David Nicol
Quoted 5 lines
Defrag works on individual files, and tries to find a contiguous
sequence of bytes to write the file's data to. In the process, the current implementation will break any CoW duplication -- either within single files (copies with cp --reflink=always) or files copied via snapshotting.While it's obviously impossible to defrag and maintain deduplicated partial chunks simultaneously without some kind of compromise (the ideal form of which is just beyond the reach of this list participant's obviousness flashlight), how difficult is it going to be to modify the current defrag implementation to hip the additional references to the file to the fact that a contiguous version (of an individual file) is now available and it's time to update their links too, after each file with multiple reflinks/snapshots to its current form has been copied to contiguous space?
Quoted 1 line
Could Bernhard Duebi do it?
Unfortunately not, I'm just a system engineer with interest in storage. I'm sorry to disapoint you.
Sincerely
Bernhard