[linux-lvm] is lvmcache really ready?

Dax Kelson dkelson at gurulabs.com
Sat Apr 23 18:10:21 UTC 2016

I don't always write emails, but when I do, it's a stream of consciousness.

We use LVM Cache in production in the infrastructure hosting our online
classroom environment. The read requests hitting our origin LV dropped by
90%. We're pretty happy with it.

I wish you could have multiple origin LVs using the same cache pool.

Dax Kelson
On Apr 23, 2016 10:54 AM, "Xen" <list at xenhideout.nl> wrote:

> Attempting to join this list, but the web interface is down.
> I am interested in using lvmcache but I hear the performance is very
> meagre, that was a thread from 2014 that I was reading. But a recent blog
> post said the same:
> https://www.rath.org/ssd-caching-under-linux.html
> This guy tries first lvmcache and performance is not noticable. Then he
> uses bcache and it works like a charm.
> He was using regular partitions on regular disks. Nothing raid going on.
> But he tried to increase boot speeds and did not notice anything.
> Setting the promote adjustments to zero made no difference.
> Is this feature defunct? I mean, is this really a usable and functional
> thing? From what it seems, I think not.
> I'll have a small mSATA SSD to test with shortly, but... if this thing is
> so bugged that it keeps reading from the origin device anyway, there's not
> much point to it.
> The reason I'm wanting to use it at this point is to speed up booting
> (although unimportant) but mostly to make a more snappy system while
> running, ie. for instance just application startup.
> At the same time that would decouple my system from my data by using that
> small SSD for the system (by way of the cache) such that system seeks do
> not any longer effect the performance of other operations on the device
> (that also holds that 'data').
> It don't mind having writeback for this because any (small) delay in
> writing to the origin would work well with this strategy.
> Some IO is buffered anyway and you might think with 8GB of RAM you could
> have some IO buffering normally, but a cache is something that buffers
> between reboots, so to speak.
> I'm simply trying to see what improvements I can get with LVM.
> That aside, I think it is annoying that on Debian systems and Ubuntu and
> Kubuntu, thin-provisioning-tools is still not included by default, and you
> also need to create that initramfs hook to have the files included into the
> initramfs. Then, Grub2 also does not support thin LVM volumes at all and
> although you can boot fine on a thin root, grub-probe will not be able to
> process these volumes, complain, and exit.
> Personally I think snapshotting on thin is much more usable, if that's
> what you want to use. No necessity to allocate sufficiently-sized volumes
> in advance. I tried to ask on #lvm but they were not helpful.
> I don't really get why LVM is being so neglected other than the popularity
> of btrfs these days.
> LVM is much more modular and very easy to work with normally. Commands
> make sense for the most part and the only thing that is missing is a nice
> gui.
> (I was even writing... some snapshot to incremental backup script at some
> point).
> LVM is one of the saner things still existing in Linux from my
> perspective, even if it doesn't feel perfect, but that is more due to the
> fact that you are using software emulation of partitions in a way that
> kinda tries to "avoid" having to do it in "firmware" -- in that sense that
> you are trying to do what regular partitions can't.
> Someone called this "deferred design" I believe.
> Because of this fact I believe it cannot really actually fully work out
> (for me) I believe particularly when it comes to cross-platformness and
> encryption as, in the same way, I prefer a firmware/bios environment for
> managing RAID arrays rather than just software.
> At the same time with UEFI et al. my opinion is just the reverse: let the
> boot loader please be software in some way. Even if it's a menu. At least
> you can adjust your software, the firmware you might not have a say about.
> Deferred design, when Linux tools do everything a regular computer
> firmware environment should. And then they created UEFI and tried to take
> stuff away that does belong to the operating system.
> While not creating anything that would work better except for GPT disks.
> I just want to ask a question though about thin LVM, but I will ask in
> another email.
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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
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