[linux-lvm] LVM Cache Configuration issues

Marian Csontos

2015-10-20 19:26:49 UTC

Post by Marc Caubet
Hi,
I am testing LVM Cache but I am completely concerned about obtained iozone
results which are worse than without using an SSD card, so my guess is that
I am doing something wrong.
RAID6(16+2)+RAID6(16+2)) & sdb (LSI Controller 2: RAID6(16+2)+RAID6(16+2)),
6TB Disks. Hence, we have a ~384TB (sda = 192TB + sdb = 192TB)
On the other hand, we purchased a 400GB SSD card, which is shown as
/dev/nvme0n1. I created 2 partitions 356.61 GiB and 16.00 GiB. I guess this
shouldn't be necessary but I created a 16GiB partition for cache metadata.
vgcreate dcvg_a /dev/sda /dev/sdb
lvcreate -i 2 -I8192 -n dcpool -l 100%PVS -v dcvg_a /dev/sda /dev/sdb
lvcreate -n cache0meta -l 100%PVS dcvg_a /dev/nvme0n1p1
lvcreate -n cache0 -l 100%PVS dcvg_a /dev/nvme0n1p2
lvconvert --type cache-pool --poolmetadata dcvg_a/cache0meta dcvg_a/cache0
vgcreate dcvg_a /dev/sda /dev/sdb
lvcreate -n dcpool -l 100%PVS dcvg_a
lvcreate --type cache -l 100%PVS -n cache0 dcvg_a/dcpool /dev/nvme0n1
- I guess both methods are the same, isn't it? The main difference is to
define or not a specific/custom partition for the pool metadata.
- Are they correct and which method is the recommended with 1 SSD? Is there
any "this is the best" recommended setup?

Hello,
LVM will allocate Logical volumes itself, no need allocating partitions.
Using 2 partitions on single drive as PVs may be *harmful* as LVM may
allocate mirror/RAID with both legs on same device.
As long as you are not using RAIDs/stripes/mirrors you are fine but it
still makes little sense.

Post by Marc Caubet
- Default mode is 'writethrough' which should be safer, but I do not see
any improvement of performance on reads and neither on writes (I used
iozone). Instead of this, performance becomes really bad. Why?

Not sure what I/O pattern is used by iozone, but you will see almost no
effects of caching doing random read/write (by design unless your cache
device is as large as the slow device which makes no sense) or long
sequential reads and writes (these can be tuned to some extent).
Only the blocks most often accessed are cached (the number of reads
should be over threshold).
See
https://www.kernel.org/doc/Documentation/device-mapper/cache-policies.txt.
Use `lvchange --cachesettings` to adjust them.

Post by Marc Caubet
- I would like to set up 'writeback', but for a single SSD (so no RAID1 for
SSD) which are the risks? I can expect that current data being written will
be lost, but no data corruption can be found in the Origin LV, isn't it?

Just the opposite. Expect corruption. If there are any filesystem
metadata written only in the cache, removing the SSD will reliably
corrupt the filesystem and I assume the FS metadata blocks being among
those most often accessed.

Post by Marc Caubet
- writethrough / writeback caches can be hot-removed from the LVM? As I can
see, it seems so.

No hot remove, not even in case of writethrough. (Work in progress.) You
would be able to recover from losing writethrough device using some
magic, but losing writeback cache will almost for sure corrupt your data.

Regards,

Martian

Post by Marc Caubet
- Any suggestions?
Thanks a lot for your help,
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