Subject: Filesystem locking and cache question.
To: None <>
From: Sung-Won Chung <>
List: tech-kern
Date: 01/17/2003 17:58:23
I am thinking a process that can relocate file system block to
be relocated other locations, to make a kind of disk defragmentation
program to get a continuous file allocation.

Now, I am thinking how to manage locking and caches to this.

1. Locking

During a relocation of a disk block, other processes should not
access that block. I think a simple solution is using a lock for
a vnode related with a block under relocation, since we can find
the inode corresponding to a block under relocation, though it
should traverse file system. This lock is different from vn_lock,
since it should protect the whole vnode operations such as

There is shortcoming in the use of locking to vnode during relocation,
since if the granularity of lock is vnode operation, using simple lock
serialize all vnode operations.  It may lead to performance degrade.
Therefore, locking must occur only when the vnodes related with a
vnode operation is under relocation, by using hash or so.

2. Cache

FFS uses inode, vnode, and buffer cache. After a block is relocated,
we should update caches related with the block just moved, before
releasing a lock that have prevented enterance of vnode operations
related with the block under relocation.

Simple solution is, instead of update, 1) flush buffer cache related
with the moved block, and 2) flush inode cache related with the moved
block, since they have old location of the block.

If we avoid cache flushing, the work to be done for inode is simple.
We just update block pointers (di_db or di_ib). However, avoiding the
flush of a buffer cache is rather complicated. 1) if the buffer cache
contains general data block, we can avoid flush only by changing
physical block number in the buffer (b_blkno). 2) if the buffer cache
contains directory entry, we can avoid flush by changing inode number
field in the relavant directory entry. 3) if the buffer cache contains
inode itself, we change file system block pointers, 4) if the buffer
cache contains a block containing indirect file system block pointers,
we updates some of those pointers to reflect new location of moved

The difficult to implement this idea is, current buffer cache doesn't
know what kind of data does it have. But adding flags that can
tell what the buffer has may degrade the file system indepedency of
buffer cache.

Thanks for any ideas about these two issues.

- Sung-Won Chung

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