rolf at rolffokkens.nl
Mon Sep 16 13:46:45 UTC 2013
Things are progressing well with the SSD cache feature, so I'd like to
give a brief update.
dm-cache & bcache
Initially both dm-cache and bcache were in scope to provide users with
the SSD caching feature in Fedora 20. Both are supported by the latest
Linux kernels, but unfortunately userspace tooling (lvm2) for dm-cache
still requires considerable effort so focus has been te have bcache in F20.
System wide or Self contained?
For F20 the intention was to have SSD cache Self contained, and to have
it System wide in F21. Looks like we're ahead of schedule! Thanks to the
great cooperation of community members responsible for other Fedora
packages, we may see a well integrated bcache implementation in F20.
Because bcache is quite new we should however consider it Experimental!
Currently I'm running my root FS on bcache. My system runs great. Both
kernel updates and other updates (in F20 updates-testing*) work without
any problems. This means dat a working intramdisk is created by dracut,
which in turn means that util-linux (blkid) detects bcache. To
'complicate' testing I have the root FS on lvm (which is on bcache)! And
as a bonus of course I'm experiencing a nice performance :-)
Because Anaconda doesn't support bcache (yet) creating a running system
with root FS on bcache requires some "manual work", but once it's
running it's running well!
*) some packages are not yet in updates testing, details below.
Thanks to Igor Gnatekno a Fedora Test day is planned 13th of october:
https://fedorahosted.org/fedora-qa/ticket/415. To support this a Wiki
page (work in progress) is maintained:
interested in testing is welcome!
libblkid in util-linux v2.24 will support bcache, which means that both
blkid and wipefs will detect (and wipe) bcache. Currently util-linux
v2.23 is available, but v2.24 rc will be in F20 (thanks Karel Zak). To
test bcache I'm running a patched v2.23 that already has the planned
support for bcache.
The bcache-tools package is available in F20 updates-testing, and seems
to be in pretty good shape (thank you reviewers, thanks Hans de Goede).
Currently bcache-tools includes a bcache-probe utility, but that may no
longer be in the package when F20 is released because it's obsoleted by
blkid in util-linux v2.24.
Dracut now has a bcache module to build initramfs(thanks Harold Hoyer).
This works very well, but it currently needs the -N option. When
util-linux v2.24 is released dracut will detect bcache and be able to
operate without the -N option.
LVM2 does normally not accept a bcache device as PV (physical volume). A
small 'one-line' patch can fix this, but the lvm2 people are working on
other lvm2 stuff. I hope Alasdair Kergon will be able to get this simple
change in LVM2.
If not, users can manually configure LVM2 to accept bcache devices as PV
by making a small change to their /etc/lvm/lvm.conf. To build a working
initramfs users need to manually pass the --lvmconf option to dracut, so
a kernel upgrade won't work out-of-the-box.
<mailto:agk at redhat.com>
I tested bcache by using a patched LVM2 that accepts bcache as a PV,
hoping the patch will be in F20.
Currently the team is very busy getting anaconda ready for F20, and
including bcache support may require considerable effort. Hopefully
anaconda has bcache support in F21.
More details: https://bugzilla.redhat.com/show_bug.cgi?id=998543
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