On Mon, Mar 21, 2011 at 09:55:07AM +0100, Andreas Dilger wrote: > Unfortunately, the overhead of allocating a whole cluster for every > index block and every directory is fairly high. For Lustre it > matters very little, since there are only a handful of directories > (under 40) on the data filesystems where this would be used and the > real directory tree is located on a different metadata filesystem > which probably wouldn't use this feature, but for most "normal" > users this overhead may become prohibitive. That is why I've been > trying to think of a way to allow sub-cluster allocations for these > uses. FYI, one filesystem I know has a neat trick that works well (IMHO). Whenever you have a file or a "tail-of-a-file" that is smaller than a cluster, you allocate it from a special file that contains all tails-of-files that are a specific size. That filesystem happened to have 64k clusters and 512byte tail-file-granularity. So it had 128 files that contained the tails of all possible differt sizes. In our case I would suggest using a sequence something like: 4k, 8k, 12k, 16k, 24k, 32k, 48k 64k, 96k, 128k and so on. That way with about 15 tail-files we are reasonably efficient (never having more than 4k or 33% allocated over the filesize). Also, adding 8k to a 128k file doesn't require moving the whole file from one spot in the filesystem to another, which would be required if we were to enumerate all 4k-multiples up to 1M. The higher bits of the block numbers will indicate that it refers to a tail-block blocknumber, and in which tail-file to look for the tail-block. Roger. -- ** R.E.Wolff@xxxxxxxxxxxx ** http://www.BitWizard.nl/ ** +31-15-2600998 ** ** Delftechpark 26 2628 XH Delft, The Netherlands. KVK: 27239233 ** *-- BitWizard writes Linux device drivers for any device you may have! --* Q: It doesn't work. A: Look buddy, doesn't work is an ambiguous statement. Does it sit on the couch all day? Is it unemployed? Please be specific! Define 'it' and what it isn't doing. --------- Adapted from lxrbot FAQ -- 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
