Re: Use fast device only for metadata?

[Nikolaus Rath <Nikolaus@rath.org>]

On Feb 07 2016, Martin Steigerwald <martin@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 <Nikolaus@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.

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..


Best,
-Nikolaus

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