Josef Bacik posted on Wed, 07 Jan 2015 15:10:06 -0500 as excerpted: >> Does this have any effect on functionality? As I understood snapshots >> still work fine for files marked like that, and so do reflinks. Any >> drawback functionality-wise? Apparently file compression support is >> lost if the bit is set? (which I can live with too, journal files are >> internally compressed anyway) >> >> > Yeah no compression, no checksums. If you do reflink then you'll COW > once and then the new COW will be nocow so it'll be fine. Same goes for > snapshots. So you'll likely incur some fragmentation but less than > before, but I'd measure to just make sure if it's that big of a deal. > >> What about performance? Do any operations get substantially slower by >> setting this bit? For example, what happens if I take a snapshot of >> files with this bit set and then modify the file, does this result in a >> full (and hence slow) copy of the file on that occasion? >> >> > Performance is the same. The otherwise nocow on-snapshot "cow1" is per-block (4096-byte AFAIK), so some fragmentation, but slower. The "perfect storm" situation is people doing automated per-minute snapshots or similar (some people go to extremes with snapper or the like...), in which case setting nocow often doesn't help a whole lot, depending on how active the file-writing is, of course. But for something like append-plus-pointer-update-pattern log files with something like per-day snapshotting, nocow should at least in theory help quite a bit, since the write-frequency and thus the prevented cows should be MUCH higher than the daily snapshot and thus the forced-block-cow1s. - FWIW, I'm systemd on btrfs here, but I use syslog-ng for my non-volatile logs and have Storage=volatile in journald.conf, using journald only for current-session, where unit status including last-10-messages makes troubleshooting /so/ much easier. =:^) Once past current-session, text logs are more useful to me, which is where syslog-ng comes in. Each to its strength, and keeping the journals from wearing the SSDs[1] is a very nice bonus. =:^) --- [1] I can and do filter what syslog-ng writes, but couldn't find a way to filter journald's writes, only queries/reads. That alone saves writes for repeated noise I'm filtering out with syslog before it's ever written, that journald would still be writing if I let it write non- volatile. -- 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 majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
