On 2017-09-19 11:30, Pat Sailor wrote:
Hello,
I have a half-filled raid1 on top of six spinning devices. Now I have
come into a spare SSD I'd like to use for caching, if possible without
having to rebuild or, failing that, without having to renounce to btrfs
and flexible reshaping.
I've been reading about the several options out there; I thought that
EnhanceIO would be the simplest bet but unfortunately I couldn't get it
to build with my recent kernel (last commits are from years ago).
As a general rule, avoid things that are out of tree if you want any
chance of support here. In the case of a third party module like
EnhanceIO, you'll likely get asked to retest with a kernel without that
module loaded if you run into a bug (because the module itself is suspect).
Failing that, I read that lvmcache could be the way to go. However, I
can't think of a way of setting it up in which I retain the ability to
add/remove/replace drives as I can do now with pure btrfs; if I opted to
drop btrfs to go to ext4 I still would have to offline the filesystem
for downsizes. Not a frequent occurrence I hope, but now I'm used to
keep working while I reshape things in btrfs, and it's better if I can
avoid large downtimes.
Is what I want doable at all? Thanks in advance for any
suggestions/experiences to proceed.
dm-cache (or lvmcache as the LVM2 developers want you to call it,
despite it not being an LVM specific thing) would work fine, it won't
prevent you from adding and removing devices, it would just make it more
complicated. Without it, you just need to issue a replace command (or
add then remove). With it, you need to set up the new cache device,
bind that to the target, add the new device, and then delete the old
device and remove the old cache target. dm-cache managed through LVM
also has the advantage that you can convert the existing FS trivially,
although you will have to take it off-line for the conversion.
A better option if you can afford to remove a single device from that
array temporarily is to use bcache. Bcache has one specific advantage
in this case, multiple backend devices can share the same cache device.
This means you don't have to carve out dedicated cache space for each
disk on the SSD and leave some unused space so that you can add new
devices if needed. The downside is that you can't convert each device
in-place, but because you're using BTRFS, you can still convert the
volume as a whole in-place. The procedure for doing so looks like this:
1. Format the SSD as a bcache cache.
2. Use `btrfs device delete` to remove a single hard drive from the array.
3. Set up the drive you just removed as a bcache backing device bound to
the cache you created in step 1.
4. Add the new bcache device to the array.
5. Repeat from step 2 until the whole array is converted.
A similar procedure can actually be used to do almost any underlying
storage conversion (for example, switching to whole disk encryption, or
adding LVM underneath BTRFS) provided all your data can fit on one less
disk than you have.
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