On Fri, 31 Oct 2014, "Peter Ross" <Petros.Listig@fdrive.com.au> wrote:
The article also mentions some speed issues especially in relation to databases.
COW filesystems use different blocks on disk every time a file is written to. For a file that is randomly written that leads to massive fragmentation which either kills linear read performance or requires online defragmentation (which hurts performance too).
I would be interested to know what Oracle says to databases on ZFS on Solaris - and Btrfs on Linux systems (the later not supported by Oracle yet, I believe, the first I am not sure about)
The same performance issues apply to BTRFS and ZFS. The significant difference is that L2ARC and ZIL can mitigate such problems - and possibly give better performance than a traditional filesystem such as XFS on a plain RAID array.
My gut feeling: Use Btrfs for "bread and butter" work and not if you need 101% reliability. With backups and mirrors and failovers (which may be in place anyway) you may be fine.
If you want good reliability then you need backups and mirrors anyway.
I just do not get my head around why a subvolumes is called subvolume if it is (according to the FAQ) comparable to a file system - you just can have many of them in a pool.
A subvolume is represented inside BTRFS in much the same way a directory. You can't mount one subvol without operating on the rest of the filesystem, so if a filesystem is corrupted such that it can only be mounted RO then that applies to all subvols. On Fri, 31 Oct 2014, Avi Miller <avi.miller@gmail.com> wrote:
Yeah, COW filesystems in general are not great for DB performance. Or VM performance. We usually recommend disabling the COW on those files (as the DB/VM product should do some form of transaction control).
I disagree. I think that it's best to have reliability features at every possible level of the stack. If you can afford the performance hit of running a database on ZFS or BTRFS then you should do it. If you are going for an all-Sun environment then the cases where the incremental cost of using COW on ZFS for a database exceed the potential benefits seem likely to be very rare. The vast majority of database installations don't require hardware that is particularly powerful by today's standards. -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/