On 2016-02-07 15:59, Martin Steigerwald wrote:
Am Sonntag, 7. Februar 2016, 21:07:13 CET schrieb Kai Krakow:
Am Sun, 07 Feb 2016 11:06:58 -0800
schrieb Nikolaus Rath <nikol...@rath.org>:
Hello,
I have a large home directory on a spinning disk that I regularly
synchronize between different computers using unison. That takes ages,
even though the amount of changed files is typically small. I suspect
most if the time is spend walking through the file system and checking
mtimes.
So I was wondering if I could possibly speed-up this operation by
storing all btrfs metadata on a fast, SSD drive. It seems that
mkfs.btrfs allows me to put the metadata in raid1 or dup mode, and the
file contents in single mode. However, I could not find a way to tell
btrfs to use a device *only* for metadata. Is there a way to do that?
Also, what is the difference between using "dup" and "raid1" for the
metadata?
You may want to try bcache. It will speedup random access which is
probably the main cause for your slow sync. Unfortunately it requires
you to reformat your btrfs partitions to add a bcache superblock. But
it's worth the efforts.
I use a nightly rsync to USB3 disk, and bcache reduced it from 5+ hours
to typically 1.5-3 depending on how much data changed.
An alternative is using dm-cache, I think it doesn´t need to recreate the
filesystem.
That's correct, dm-cache can use a regular underlying storage device.
This of course has potential implications for a multi-device filesystem
(it can seriously confuse BTRFS and cause data corruption), but it works
just fine for a single device filesystem. This makes it a bit easier to
test run, but also means you need more devices (internally, it uses 3,
one backing device, one cache device, and a metadata device for
persistently mapping between the two). It's really easy to set up
though if you have a recent version of LVM built with dm-cache support.
In general, bcache takes a bit more setup, but avoids the multi-device
issues, and importantly, doesn't require LVM or dmsetup (which are
usually pretty big packages on many distros). The caveat with bcache
though is that there have been issues in the past with data integrity
when used with BTRFS, but if you're on a recent kernel (at least 4.0 if
you're using BTRFS for actual data storage), you should have no issues.
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