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Update comment to explain search order reverted to historical order

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in -r254996.

Suggested by: Pedro Giffuni <pfg@FreeBSD.org>
MFC:          3 days
This commit is contained in:
Kirk McKusick 2014-03-22 11:26:39 +00:00
parent 44f1c91610
commit e24784d32a
1 changed files with 11 additions and 10 deletions
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21
sys/ufs/ffs/ffs_alloc.c
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@ -1167,16 +1167,17 @@ ffs_dirpref(pip)
* We scan from our preferred cylinder group forward looking
* for a cylinder group that meets our criterion. If we get
* to the final cylinder group and do not find anything,
* we start scanning backwards from our preferred cylinder
* group. The ideal would be to alternate looking forward
* and backward, but that is just too complex to code for
* the gain it would get. The most likely place where the
* backward scan would take effect is when we start near
* the end of the filesystem and do not find anything from
* where we are to the end. In that case, scanning backward
* will likely find us a suitable cylinder group much closer
* to our desired location than if we were to start scanning
* forward from the beginning of the filesystem.
* we start scanning forwards from the beginning of the
* filesystem. While it might seem sensible to start scanning
* backwards or even to alternate looking forward and backward,
* this approach fails badly when the filesystem is nearly full.
* Specifically, we first search all the areas that have no space
* and finally try the one preceeding that. We repeat this on
* every request and in the case of the final block end up
* searching the entire filesystem. By jumping to the front
* of the filesystem, our future forward searches always look
* in new cylinder groups so finds every possible block after
* one pass over the filesystem.
*/
prefcg = ino_to_cg(fs, pip->i_number);
for (cg = prefcg; cg < fs->fs_ncg; cg++)