On 2011-09-20, at 9:29 AM, torn5 wrote: > On 09/20/11 14:47, Theodore Tso wrote: >> But that's OK, because I don't know of any RAID array that supports this kind of radical surgery in parameters in the first case. :-) > > Ted, thanks for your reply, > > Linux MD raid supports this, it's called reshape. Most parameters changes are supported, in particular the addition of a new disk and restriping of a raid5 is supported *live*. It's not very stable though... > > But apart from the MD live reshape/restripe, what I could do more likely is to move such filesystem *live* across various RAIDs I have, leveraging LVM's "pvmove". Such RAIDs are almost all of 1MB stride, but with various number of elements, hence they have a different stripe-width. Just FYI, we use 1MB stripe width for Lustre by default, and this is large enough for very good IO performance, and is very well tested. >> The other thing to consider is small writes. If you are doing small writes, a large stripe size is a disaster, because a 32k random write by a program like MySQL will turn into a 3MB read + 3MB write request. > > So, regarding my original problem, the way you use stride-size in ext4 is that you begin every new file at the start of a stripe? With ext4 and flex_bg it is nearly irrelevant. I try to use a flex_bg size that is -G 256 to match the stripe_width, so that the bitmap load on all the disks is even. It would be good to have a patch to do this by default, but I haven't gotten around to that. > For growing an existing file what do you do, do you continue to write it from where it was, without holes, or you put a hole, select a new location at the start of a new stripe and start from there? Large files will be allocated at the start of the stripe_width (1MB) alignment, and (IIRC) small files will be packed together into a 1MB chunk, to minimize read-modify-write on the RAID. > Regarding multiple very small files wrote together by pdflush, what do they do? They are sticked together on the same stripe without holes, or each one goes to a different stripe? I'm not 100% sure that the small files are still being handled correctly, since it is a long time since I looked at that code and it has undergone a lot of changes. > Is the change of stripe-width with tune2fs supported on a live, mounted fs? (I mean maybe with a mount -o remount but no umount) I think it is cached in the in-memory superblock to include any mount-time parameters and sanity checks, so I don't think it will take effect. 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
