On 2016-02-08 08:20, Qu Wenruo wrote:
On 02/08/2016 08:24 PM, Austin S. Hemmelgarn wrote:
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.
And I just want to add more about using a device *only* for metadata.
The short answer is, unfortunately, NO.
1) Even using bcache/dm-cache, it may still cache small data write
Although I'm not quite sure about dm-cache/bcache, but as long as the
top file is Btrfs, it won't be possible to limit data/metadata to/from
specific device.
IIRC, bcache or similiar method may cache most random r/w of metadata,
it's still quite possible to cache a lot of random r/w of data.
And depending on the sector size(minimal data block size) and leaf size
(metadata block size), it's even more possible to cache small data other
than metadata under specific worload.
As default sectorsize is 4K, but leafsize is 16K.
The mention of dm-cache/bcache was more intended as an alternative,
since BTRFS currently can't do what Nikolaus was trying to achieve.
Neither will give quite the performance profile that a dedicated
metadata device might, but they should still significantly improve
general performance. In essence, these function for BTRFS like L2ARC on
an SSD does for ZFS.
2) Btrfs don't have special preference on chunk allocation.
Btrfs just allocate chunks in the order of unallocated space.
So, even there is a super big TB or PB spinning device, and GB level
SSD, btrfs will just trust them according to unallocated space.
On at least the project page, there is a suggestion to provide this
functionality. In a way, it's essentially equivalent to the external
journal device supported by ext4, XFS, OCFS2 and some other filesystems,
and as such, I'd say it's a feature we should seriously consider looking
at implementing eventually, even if just for feature parity, and even if
we speed up metadata operations in BTRFS.
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