[dm-devel] question regarding snapshot chunk size.
Kevin Corry
kevincorry at sbcglobal.net
Fri May 14 00:19:51 UTC 2004
On Thursday 13 May 2004 18:30, Dave Olien wrote:
> I'm using your udm1 patch set to linux 2.6.6-rc2.
> The man page for lvcreate says the chunksize parameter for a
> snapshot can be a power of two value between 4k and 1024k.
>
> When I try --chunksize 1024, I hit a BUG() in the bvec_index()
> function.
>
> Putting some checking code into snapshot_ctr(), I observe that
> when the lvcreate --chunksize 1024 parameter is used, snapshot_ctr()
> reports a chunksize of 2048. This is the value just after the
> simple_strtoul() call.
>
> So, why is the value being doubled (by the lvcreate command?).
The value isn't being doubled. dm-snapshot expects the chunk-size value in
sectors, whereas the user-space tools present the option in kilobytes.
So....I guess the value *is* being doubled, but for the right reason.
> Also, why is there no range check on the value in snapshot_ctr(),
> with a error code failure, rather than a BUG() call in bvec_index?
So it seems there are some implicit limits when using the dm-io APIs. For the
time being, we might need to add a size check in dm-snapshot to prevent
chunk-sizes greater than 512kB. We should also review the dm-io code to see
if these limitations are real, or if there's a way to get around them.
--
Kevin Corry
kevincorry at sbcglobal.net
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