I was looking at the /sys/fs/ext4/*/lifetime_write_kbytes counters on my home server and wondering about how accurate they are. That is most interesting in the case of flash devices, to get a good idea of the lifetime writes vs. actual rated drive writes per day. It looks like s_kbytes_written is only updated on clean unmount via ext4_commit_super->ext4_update_super() and in a few error handling codepaths. This means any in-memory updates are typically lost if the server crashes or loses power (which is typical for long-running servers, rather than a clean shutdown). It would be useful to periodically update the superblock with the current value, maybe once an hour if the value has changed more than some small margin (to take into account the *previous* update). The superblock used to be written frequently via ->write_super(), but this has not been the case since commit v3.5-rc5-19-g4d47603d9703. Any thoughts/objections to a periodic task calling ext4_update_super() every hour if there have been any noticeable writes since the last time it was called? This could potentially be more clever so that it only writes if the disk is not asleep, and do the writes the next time it wakes, but I'm not sure how easy/hard that is to detect at the filesystem level. Cheers, Andreas PS: there is *also* a function resize.c::ext4_update_super() for added confusion, but that does something completely different...
Attachment:
signature.asc
Description: Message signed with OpenPGP
