Christoph Anton Mitterer posted on Thu, 26 Nov 2015 19:25:47 +0100 as excerpted:
> On Thu, 2015-11-26 at 16:52 +0000, Duncan wrote: >> For people doing snapshotting in particular, atime updates can be a big >> part of the differences between snapshots, so it's particularly >> important to set noatime if you're snapshotting. > What everything happens when that is left at relatime? > > I'd guess that obviously everytime the atime is updated there will be > some CoW, but only on meta-data blocks, right? Yes. > Does this then lead to fragmentation problems in the meta-data block > groups? I don't believe so. I think individual metadata elements tend to be small enough that several fit in a metadata node (16 KiB by default these days, IIRC), so there's no "metadata fragmentation" to speak of. > And how serious are the effects on space that is eaten up... say I have > n snapshots and access all of their files... then I'd probably get n > times the metadata, right? Which would sound quite dramatic... > > Or is just parts of the metadate copied with new atimes? I think it's whole 4 KiB blocks and possibly whole metadata nodes (16 KiB), copy-on-write, and these would be relatively small changes triggering cow of the entire block/node, aka write amplification. While not too large in themselves, it's the number of them that becomes a problem. IIRC relatime updates once a day on access. If you're doing daily snapshots, updating metadata blocks for all files accessed in the last 24 hours... Again, individual snapshots aren't so much of a problem, and if you're thinning to the 250 snapshots per subvolume or less as I recommend, the problem will remain controlled, but at 250, starting at daily snapshots so they all have atime changes for at least all files accessed during that 24 hours, that's still a sizable set of unnecessarily modified and thus space-taking snapshotted metadata. But I wouldn't worry about it too much if you're doing say monthly snapshots and only keeping a year's worth or less, 12-13 snapshots per subvolume total. In my case, I'm on SSD with their limited write cycles, so while the snapshot thing doesn't affect me since my use-case doesn't involve snapshots, the SSD write cycle count thing certainly does, and noatime is worth it to me for that alone. -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
