[dm-devel] dm-cache with zero hit rate

[Steinar H. Gunderson]

Hi,

I recently set up dm-cache below my main LVM, so that /dev/md2 (a RAID-0 of
two SSDs) has an LVM with /dev/mapper/cache-blocks and
/dev/mapper/cache-metadata, which then back /dev/md1 (a RAID-6 of rotating
disks), where my main LVM, including the root device etc., lies.

After a fair amount of fighting with udev and initramfs-tools, plus upping
the block size to 2048 since 512 complained about not enough RAM (on a 24GB
machine!), this seems to boot up and work, but I seem to get absolutely zero
cache hits.

dmsetup table for the device:

  cache: 0 23440891904 cache 254:0 254:1 9:1 2048 1 writeback default 4 random_threshold 8 sequential_threshold 512

dmsetup status for it:

  cache: 0 23440891904 cache 913/8192 0 170976 0 9614 0 0 0 0 0 2 migration_threshold 2048 4 random_threshold 8 sequential_threshold 512

Note in particular 0/170976 cached reads and 0/9614 cached writes.

Is this normal? Is there any good reason why an LVM on top of a dm-cache
device would not be supported? Or do I just need to wait a few more
hours/weeks/days until it starts being aggressive?

/* Steinar */
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Homepage: http://www.sesse.net/