It is whispered that Greg Freemyer was heard, on or about 1/28/2004
12:19 PM to say:
A snapshot is based on copy on write technology. I guess you could use
it to create the transaction log you are talking about, but it is not
what it is designed to do, and I believe significant effort would be
involved.
Basically with a snapshot, a blank disk volume is created with a flag
table with one entry per block of master volume data. All of the flag
entries are initialized as "unmodified".
When a write is performed to the master volume, the flag table is
checked. If the flag for that particular block is set to "unmodified",
then a copy of the unmodified block is made to the snapshot area prior
to the write being performed. The flag is then set to modified.
Future writes to that specific block will have no effect on the flag
table, nor on the snapshot volume.
Oh well, I guess I was thinking that it was the reverse: changes were
written to the new volume, which was then merged into the old one once
the snapshot was finished.
For your transaction log, the flag table is significant, but the actual
snapshot volume is not. It is just the old unmodified content.
To get a true set of differences over a 24-hour period, you would have
to create a snapshot at time T, and another at time T+24. (Easily done
I think).
Then have code that went thru the flag table for snapshot T and
retrieved the blocks from snapshot T+24 that were marked as modified.
(LVM will automatically get the block from the appropriate source: T+24
or Master volume)
Once done, the snapshot T would be deleted, and snapshot T+24 would be
maintained. At the 48 hour mark, snapshot T+48 would be created and the
process repeated.
I don't think that is a trivial issue, and I think the code would have
to implemented in the kernel.
Why the kernel? ... surely reading the flag tables could be done in user
space? I think you have the necessary logic spot on.
New code would also have to be developed to apply the above blocks.
Hmmm. I can mess around in perl and java as well as the next guy, but
my c-fu is weak, very weak. How much would it cost to sponsor someone
to do this do you think?
--
Chris Beck / Y.A.B.A. / Fungal Genomics
CFSG / Concordia University
"La loi dans sa majestueuse ÃgalitÃ, interdit à tous, aux riches comme
aux pauvres de dormir sous les ponts, de coucher dans la rue et de voler
du pain." -- Anatole France (Les Lys Rouge - 1894)
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