[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]

Re: [dm-devel] LVM snapshot broke between 4.14 and 4.16



Dne 4.8.2018 v 07:20 Theodore Y. Ts'o napsal(a):
On Fri, Aug 03, 2018 at 03:30:37PM -0400, Mike Snitzer wrote:

I was trying to give context for the "best to update lvm2 anyway"
disclaimer that was used.  Yeah, it was specious.

Well, it seemed to indicate a certain attitude that both Linus and I
are concerned about.  I tried to use more of a "pursuading" style to
impress why that attitude was not ideal/correct.  Linus used a much
more assertive style (e.g., "Hell, no!").

My whole point was - when someone has 'enough time' to compile fresh new kernel, spending couple seconds to build also recent lvm2 should have be no issue. Bugs are continually fixed, new feature from newer kernel are being
used, known problems are being addressed - that's  all what it means.
Bug are fixed not just in kernel, but also in user-space.


And yeah, that isn't a good excuse to ignore it but: dm-snapshot is a
steaming pile as compared to dm thin-provisioning...

On a side note, this is the first that I've heard the assertion that
dm-thin was better than dm-snapshot.  My impression was that
dm-snapshot was a proven code base, that only did one thing and (as
far as I could tell) did it well.  In contrast, dm-thin is much newer
code, **far** more complex, with functionality and corner cases
approaching that of a file system --- and just to be even more
exciting, it doesn't have an fsck/repair tool to deal with corrupted
metadata.

In your opinion, is it because you disagree with the assumption that
dm-thin is scary?  Or is the argument that dm-snapshot is even
scarier?


dm-snapshost has really outdated design - it's been useful in the old age where megabyte was hell lot of space.

Nowadays, when users do need to handle snapshots in multi gigabyte sizes and moreover have number of snapshots from the same volume taken over the time, want to take snapshot of snapshot of snapshot, the old snapshot simple kills all the performance, uses tons of resources and becomes serious bottleneck of your system and has lots of usability limitation.

That's where  thin provisioning will shine....

However if you still need to take short-living smallish snapshot and you want to use non-provisioned origin device, there the snapshot will still work as proven technology - just no one can expect it will scale for the recent device size explosion...

Regards

Zdenek


[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]