[linux-lvm] Using snapshots for transactions
shane at hathawaymix.org
Fri Mar 4 22:41:31 UTC 2005
My company is working on a system for storing a lot of data, for a long
time, at low cost. It will involve thousands of hard drives and error
One piece we need in this system is a way to impose transaction
semantics on block devices. I've been looking everywhere for a good way
to do this. I was happy to discover today that the snapshot
functionality in Linux's device mapper code comes very close to what I
need. Now I'm looking for assistance on figuring out how to get the
rest of the way there.
Using LVM, I'd like to create a logical volume that consumes most of the
space on a hard drive. I will create a filesystem on the volume, but I
will not mount it. Then I'd like to create a snapshot of that volume
and mount the snapshot read-write. LVM and device-mapper do all of this
Now here are the parts I'm not sure how to do:
1) Just before committing, I need to freeze the snapshot and read the
"before" and "after" versions of all blocks changed. The delta will be
used to update error correction codes stored on other servers.
2) I need to atomically commit the changes I've made to the snapshot
back to the original volume.
3) If something goes wrong during commit (power failure, etc.), I need
to retry the commit. (If the retry fails, I need to disable the volume.)
It's OK that this strategy results in extra I/Os. Most of the work of
committing a transaction can probably be done in user space if I can
read the internal structures of the snapshot. Also, I'd like a way to
clear the snapshot's exception list once the transaction commits; that
way I don't have to destroy and create snapshots for each transaction.
So, am I going in a good direction? Does anyone have ideas, hints, or
advice? I can hack the kernel if I need to.
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