On Tue, August 11, 2009 10:39, Ed Spencer wrote:
> I suspect that if we 'rsync' one of these filesystems to a second
> server/pool that we would also see a performance increase equal to what
> we see on the development server. (I don't know how zfs send a receive
Rsync has to traverse the entire directory tree to stat() every file to
see if it's changed (and if it has, it then computes which parts of the
file that have been updated). Zfs send/recv however works at a lower level
and doesn't go to each file: it can simply compare which file system
blocks have changed.
So you would create a snapshot on the ZFS file system(s) of interest and
send it over to where ever you want to replicate it. Later on you would
create another snapshot and, with the incremental ("-i") option in
zfs(1M), you could then only transfer the blocks of data that were changed
since the first snapshot. ZFS will be able to figure out the block
differences without having to touch every file.
Two pretty good explanations at:
http://www.markround.com/archives/38-ZFS-Replication.html
http://www.cuddletech.com/blog/pivot/entry.php?id=984
> work so I don't know if it would address this "Filesystem Entropy" or
> specifically reorganize the files and directories). However, when we
> created a testfs filesystem in the zfs pool on the production server,
> and copied data to it, we saw the same performance as the other
> filesystems, in the same pool.
Not surprising, since any file systems on any particular pool would be
using the same spindles. If you want different I/O characteristics you'd
need a different pool with different spindles.
> We will have to do something to address the problem. A combination of
> what I just listed is our probable course of action. (Much testing will
> have to be done to ensure our solution will address the problem because
> we are not 100% sure what is the cause of performance degradation). I'm
Don't forget about the DTrace Toolkit, as it has many handy scripts for
digging into various performance characteristics:
http://www.brendangregg.com/dtrace.html
> But please, don't miss the point I'm trying to make. ZFS would benefit
> from a utility or a background process that would reorganize files and
> directories in the pool to optimize performance. A utility to deal with
If you have a Sun support contract call them up and ask for this
enhancement. If there's enough people asking for it the ZFS team will add
it. Talking on the list is one thing, but if there's no "official" paper
trail in Sun's database, then it won't get the attention it may deserve.
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