On Thu, Dec 13, 2012 at 04:17:53PM -0800, Darrick J. Wong wrote:
> Mmmm, another one! I've been looking forward to this.... :)
> > +dm-cache is a device mapper target written by Joe Thornber, Heinz
> > +Maueslhagen, and Mike Snitzer.
>
> Is this "Mauelshagen"?
y
>
> > +- Migration - Movement of a logical block from one device to the other.
> > +- Promotion - Migration from slow device to fast device.
> > +- Demotion - Migration from fast device to slow device.
>
> If a block is promoted to the fast device, is it always the case that the block
> still resides on the slow device? The existence of WT mode implies this, but
> on the other hand you do say "move" here.
>
> Or is this more like tiered storage where promoting a block moves it to the
> fast device and it's no longer on the slow device?
>
> Put another way -- if I use my SSD as a writethrough cache for a disk and one
> day the SSD loses its brains, can I expect to still have a reasonably up to
> date copy on the disk?
The cached device is the same size as the origin (not origin + ssd).
If writethrough mode is selected then writes will not complete until
they've hit both the origin and the cache. This makes it a relatively
safe way to try the cache out.
Here's the little test I use to demonstrate this:
def test_writethrough
size = gig(2)
# wipe the origin to ensure we don't accidentally have the same
# data on it.
with_standard_linear(:data_size => size) do |origin|
wipe_device(origin)
end
# format and set up a git repo on the cache
with_standard_cache(:format => true,
:io_mode => :writethrough,
:data_size => size) do |cache|
git_prepare(cache, :ext4)
end
# origin should have all data
with_standard_linear(:data_size => size) do |origin|
git_extract(origin, :ext4, TAGS[0..1])
end
end
> How do I calculate how big the metadata device has to be?
Metadata size will be proportional to the number of blocks in your
cache (ssd_size / block_size). For each block we store:
- The mapping and some flags (64bits)
- Policy hint data (size determined by policy, 64bit for mq)
In addition we store a small bitset that records the discard state of
the origin.
A good rule of thumb would be: 4mb + (16bytes * nr_blocks)
> Is there any documentation for the message formats?
>
> Or the policy parameters?
Not yet, I'll let Heinz handle this, he put the messaging stuff in.
- Joe
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