On 2013-05-19, at 8:00, Theodore Ts'o <tytso@xxxxxxx> wrote: > On Fri, May 17, 2013 at 06:51:23PM +0200, frankcmoeller@xxxxxxxx wrote: >> - Why do you throw away buffer cache and don't store it on disk during umount? The initialization of the buffer cache is quite awful for application which need a specific write throughput. >> - A workaround would be to read whole /proc/.../mb_groups file right after every mount. Correct? > > Simply adding "cat /proc/fs/<dev>/mb_groups > /dev/null" to one of the > /etc/init.d scripts, or to /etc/rc.local is probably the simplest fix, > yes. > >> - I can try to add a mount option to initialize the cache at mount time. Would you be interested in such a patch? > > Given the simple nature of the above workaround, it's not obvious to > me that trying to make file system format changes, or even adding a > new mount option, is really worth it. This is especially true given > that mount -a is sequential so if there are a large number of big file > systems, using this as a mount option would be slow down the boot > significantly. It would be better to do this parallel, which you > could do in userspace much more easily using the "cat > /proc/fs/<dev>/mb_groups" workaround. Since we already have a thread starting at mount time to check the inode table zeroing, it would also be possible to co-opt this thread for preloading the group metadata from the bitmaps. >> - I can see (see debug output) that the call of ext4_wait_block_bitmap in mballoc.c line 848 takes during buffer cache initialization the longest time (some 1/100 of a second). Can this be improved? > > The delay is caused purely by I/O delay, so short of replacing the HDD > with a SSD, not really.... Well, with a larger flex_bg factor at format time there will be more bitmaps allocated together on disk, so fewer seeks needed to load them after a new mount. We use a flex_bg factor of 256 for this reason on our very large storage targets. Cheers, Andreas-- 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
