On 3/30/13 7:49 AM, Stan Hoeppner wrote: > On 3/19/2013 5:14 AM, Stan Hoeppner wrote: >> On 3/19/2013 3:24 AM, Martin Steigerwald wrote: > ... >>> Heck, even I was confused at first. Cause the manpage of fsck.ext4 IMHO is >>> not really clear about that topic to say the least. I tested it out for a >>> reason. >> >> I already contacted Ted off list hoping he can point me to the relevant >> kernel documentation, so I don't make such a mistake again with EXT. > > Ok, so here's the skinny on the source of our confusion WRT how/when > EXT4 replays journals, and it's rather interesting. Ted Ts'o explained > the following. Where was this, out of curiosity? > The EXT4 kernel module does have code to perform journal replay, but it > is rarely executed. The reasons for this are: > > 1. EXT4 journal replay can take a lot of time (whereas XFS is instant) > 2. EXT4 systems tend to have multiple filesystems, often one per drive > (whereas XFS systems tend to have few filesystems) Those are, I think, gross generalizations. Journal replay takes as long as it takes to replay all the IO required, which can vary greatly. And TBH I have no idea where the notion came from that systems have many ext4 filesystems but few xfs filesystems. > 3. Linux mounts filesystems serially during startup I think that is correct. > To prevent potentially lengthy boot times, the init scripts run e2fsck > to replay all EXT4 filesystem journals in parallel, well before the > mount stage. I'd never heard this rationale before, but I could believe that maybe parallel log replays from userspace are faster, although it probably depends a lot on how many spindles are available to do the work - fsck avoids running in parallel for filesystems on the same physical disk, at least according to the manpage. > Thus the only case where the EXT4 kernel module performs > journal replay is when doing a mount while the system is running, e.g. > USB hard drive. Or when running xfstests ;) Technically, it does replay when the kernel mount code finds a dirty log. That's interesting, though, I hadn't thought about how most systems probably don't get a ton of coverage of kernelspace ext[34] log replay. > There are other reasons e2fsck was chosen to perform journal replay at > boot in addition to the speed issue, but as I understood Ted this is the > main reason. Ok, I can see some rationale to parallel userspace log replays; it'd be interesting to actually measure that result, though. -Eric _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
