Re: Reviewing ext3 improvement patches (delalloc, mballoc, extents)

[Alex Tomas <alex@clusterfs.com>]

>>>>> Werner Almesberger (WA) writes:

 WA> Do you plan to reserve space as "blocks, somewhere", or as "these
 WA> specific on-disk locations" ? In ABISS, we did something of the
 WA> latter kind (in order to make large contiguous allocations also on
 WA> FAT), and it turned out to be a big mess, because ABISS needed too
 WA> much support from the file system driver. So we just scrapped that
 WA> bit :-)

I see no reason to reserve specific block in ->prepare/->commit in
delayed allocation case. We already do this with reservation.
The sole point of delayed allocation is to allocate many blocks at once:
to minimize fragmentation, to decrease allocator involvement, to avoid
allocation at all if the file gets truncated quickly.

 WA> The main parts: we added a new page flag, PG_delalloc, which
 WA> basically tells everyone to stay away from that page. There are
 WA> two purposes: (a) to make sure no allocation happens unless
 WA> explicitly requested, and (b) prevent the page from being written
 WA> back while it is still in ABISS' playout buffer. The reason for
 WA> (b) is that the page gets locked during writeback, which could
 WA> cause delays if the ABISS-using application then decides to
 WA> access the page.

locked during writeback? PG_writeback should be used instead of PG_locked.


thanks, Alex



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