Hi Darrick, Somehow I missed your posts in November. You're right, the smaller is the chunk size the larger is the metadata - standard dedup problem. The hope is that one recuperates the loss of metadata space on better data deduplication ratio, which grows as the chunk size decreases. I find having an "external" B-Tree somewhat more reasonable than in-line metadata in dm-dedup (situation is different for dm-checksum, though). In dm-dedup we have to allocate blocks on-demand, keep reference counters, and garbage collect unused blocks, etc. DM's persistent-data library on which we rely helps with this enormously. Vasily On Wed, Nov 26, 2014 at 7:39 PM, Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote: > On Wed, Nov 26, 2014 at 07:07:22PM -0500, Mike Snitzer wrote: >> On Wed, Nov 26 2014 at 6:47pm -0500, >> Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote: >> >> > Sigh... >> > >> > Well, I wrote up a preliminary version of dm-checksum and then >> > realized that I've pretty much just built a crappier version of >> > dm-dedupe, but without the dedupe part. Given that it stores >> > checksums in a btree which claims to be robust through failures and >> > gives us automatic deduplication, I wonder if it we could achieve our >> > aims by modifying dm-dedupe to verify the checksums on the read path? >> > >> > I guess it would be interesting to see how bad the performance hit is >> > with the online dedupe part enabled or disabled. dm-dedupe v2 went >> > out on the mailing list last August, which I missed. :( >> > >> > Unless... there's a specific reason nobody mentioned dm-dedupe here? >> >> As you may have seen in the dm-dedup thread, we need to actively >> review/test that target > > It was in fact today's exchange on that thread that made me slap myself on > the forehead and utter "D'oh!". > >> (if your initial review focus is on extending it >> to _optionally_ verify the checksums on the read path then so be it). > > Yes, sorry, I meant to say "optionally to verify" in there. Adding a minor > feature like that might be a good check to make sure I actually understand > what's going on. :) > >> See: https://www.redhat.com/archives/dm-devel/2014-November/msg00114.html >> Specifically, the git branch that builds on v2 based on my initial >> review of v2: >> >> git://git.fsl.cs.stonybrook.edu/scm/git/linux-dmdedup >> branch: dm-dedup-devel >> >> Your help on getting dm-dedup upstream would be very much appreciated. > > <-- reading the OLS paper, as a start. What happens to the metadata btree if > someone sets the chunk size to 4KB? Will it become ungainly huge? The thing > that I wrote simply wrote a block's worth of checksums inline with the data, > which required a certain amount of slicing and dicing of bios but wasn't too > horrible with performance. > > --D > >> >> Thanks, >> Mike >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-ext4" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html > > -- > dm-devel mailing list > dm-devel@xxxxxxxxxx > https://www.redhat.com/mailman/listinfo/dm-devel > -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
