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Forum Discussion
6.2.0 has no manual to download
There is no manual for the new firmware 6.2.0 release. A manual is needed to explain in better detail the new features, how to use them and what they are. I have several question in regards to 6...
Which both sort of make exactly my point. :) A DB or a large VM disk file will have a lot of random updates. CoW /isn't/ just a matter of what happens in snapshots, though that will certainly contribute. BtRFS (like ZFS) does CoW *in the filesystem itself*, regardless of snapshots. :) Now, for most Linux distros, that is enabled by default. Something that's a bit confusing in the settings for shares now is "BitRot protection (COW)"... so I presume this is our way of changing CoW on the share? Because it seems to be enabled separately from the snapshot schedule. (Note: This is where DOCUMENTATION is handy.)
And to mdgm's point, YES, that is an excellent example of a serial write of a large file (assuming you aren't, as StephenB pointed out, downloading it via bittorrent... which will cause blocks of the file to be written in random order and likely not preallocated. I don't know if most torrent clients allow for this, but it's most efficient to pre-allocate the file in one step and then write into at random locations and have CoW disabled. But at least if you can't preallocate and disable CoW, writing once is infinitely better than having a large file that you do random write I/O to (like DB or VM disk files).
It's a question of understand how different patterns of I/O impact the filesystem structures when CoW is active. ZFS has features that can rebalance as I/O is happening (rather than running a batch process to rebalance). But unless you're expecting particular performance out of the thing, CoW is probably not going to be hugely noticeable.
There were definitely some performance problems in earlier 6.x builds with snapshots. I have slowly re-enabled them on shares with 6.1.8 and 6.1.9 and found them to much more solid now. Since the upgrade to 6.2, all my pre-existing shares seem to reflect the expected CoW default with BtRFS. At least it's nice to know we can defeat it if desired. I do wonder how it behaves on an existing share if you disable it. The usual case is that you have to make new copies of every pre-existing file (once +COW is set) in order to properly disable it. Does the NAS do that? If not, it might be "better" to only let that option be flipped until there are files in the share (to make it clear that CoW isnt fully disabled in that case)...?
Anyway, whether one choosed to disable it (with or without snapshots) depends entirely on the workload and performance expectations. Most people will never know.
And to mdgm's point, YES, that is an excellent example of a serial write of a large file (assuming you aren't, as StephenB pointed out, downloading it via bittorrent... which will cause blocks of the file to be written in random order and likely not preallocated. I don't know if most torrent clients allow for this, but it's most efficient to pre-allocate the file in one step and then write into at random locations and have CoW disabled. But at least if you can't preallocate and disable CoW, writing once is infinitely better than having a large file that you do random write I/O to (like DB or VM disk files).
It's a question of understand how different patterns of I/O impact the filesystem structures when CoW is active. ZFS has features that can rebalance as I/O is happening (rather than running a batch process to rebalance). But unless you're expecting particular performance out of the thing, CoW is probably not going to be hugely noticeable.
There were definitely some performance problems in earlier 6.x builds with snapshots. I have slowly re-enabled them on shares with 6.1.8 and 6.1.9 and found them to much more solid now. Since the upgrade to 6.2, all my pre-existing shares seem to reflect the expected CoW default with BtRFS. At least it's nice to know we can defeat it if desired. I do wonder how it behaves on an existing share if you disable it. The usual case is that you have to make new copies of every pre-existing file (once +COW is set) in order to properly disable it. Does the NAS do that? If not, it might be "better" to only let that option be flipped until there are files in the share (to make it clear that CoW isnt fully disabled in that case)...?
Anyway, whether one choosed to disable it (with or without snapshots) depends entirely on the workload and performance expectations. Most people will never know.
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