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Fri, 15 Jun 2007

NILFS by AMAGAI Yoshiji at Nippon Telegraph and Telephone Corporation.


NILFS is log-structured filesystem by Japan telecom company, which allows to have snapshots and garbage collection over them.
Looks interesting, but it has fundamental flaw - it writes all metadata updates as a log in a special file, which means heavy fragmentation when oldest updates are removed. It also supports (at least it looks so) copy-on-write (and it looks like only for data), which allows to forget about slow recovery after crash and can improve performance.
NILFS supports checksums for segment data and related metadata. Superblock and management blocks maintain own checksums. Currently it is only crc32. There are no checksums for individual data blocks.
NILFS uses simple B-tree structure with 64-bit keys.
Like Btrfs it does not support sync, direct and mmapped processing and quotas.
It also does not allow to have writable snapshots.

First papers about his filesystem appeared two years ago.
There are no benchmarks on the official site.

It looks like log-structured filesystems become popular (getting into accout when I first time wrote about such filesystems in this blog there were zero projects in that direction).

/devel/fs :: Link / Comments (0)

BTRFS by Chris Mason at Oracle.


There are number of features currently not implemented, and which limits design:

• Absence of delayed allocation.
This means that fragmentation will kill a filesystem, even if it is extent based (which is just a block size increase). Currently btrfs allocates new on-disk chunk for each 8k write.

• Bad tree locking
Which does not allow multithreaded writing.

• Some minor nits like absence of sync, direct and mapped processing, which is unlikely to be design problem though.

/devel/fs :: Link / Comments (0)