On 2016-02-08 16:44, Nikolaus Rath wrote:
On Feb 07 2016, Martin Steigerwald <mar...@lichtvoll.de> 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.
Yes, I tried that already but it didn't improve things at all. I wrote a
message to the lvm list though, so maybe someone will be able to help.
That's interesting. I've been using BTRFS on dm-cache for a while, and
have seen measurable improvements in performance. They are not big
improvements (only about 5% peak), but they're still improvements, which
is somewhat impressive considering that the backing storage that's being
cached is a RAID0 set which gets almost the same raw throughput as the
SSD that's caching it. Of course, I'm using it more for the power
savings (SSD's use less power, and I've got a big enough cache I can
often spin down the traditional disks in the RAID0 set), and I also
re-tune my system as hardware and workloads change, and my workloads
tend to be atypical (lots of sequential isochronous writes, regular long
sequential reads, and some random reads and rewrites), so YMMV.
Otherwise I'll give bcache a shot. I've avoided it so far because of the
need to reformat and because of rumours that it doesn't work well with
LVM or BTRFS. But it sounds as if that's not the case..
It should work fine with _just_ BTRFS, but don't put any other layers
into the storage system like LVM or dmcrypt or mdraid, it's got some
pretty pathological interactions with the device mapper and md
frameworks still.
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