Re: adding linux syscall fallocate

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

> Now, what happens if you do a posix_fallocate("foo", 0, 4GiB)?  The file sys$

Given the intent of posix_fallocate as I understand it, that's the only
reasonable option for FFS.  If a filesystem has a way to record a block
that should read as zeros but is allocated in the sense that a block is
guaranteed to be available to back writes, then, maybe.  But are there
any such?  (See below.)

> Technically, a file system is allowed to make that optimization for the "all$

Speaking from an FFS perspective....

All you need is a second magic block number.  Block number zero is
already reserved for holes.  Making, say, block number 1, or -1, or
some such, reserved to represent "block-of-zeros semantics for which
all backing data has been accounted as allocated", so that writing such
a block is guaranteed to have space available?  That sounds like a way
to implement that that's relatively simple and involves relatively
small changes.  (Indirect blocks would complicate it slightly, but not
all that much.)  Of course, it would be incompatible with pre-change
implementations, but I think that has to be true of any such
optimization, since it has to involve an on-disk change.

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