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SubjectRe: Raw-io & Swap partitions
Hi Peter.

> There seem to be fairly significant parallels between Linux's
> support of special partitions for swap, and the arguments for raw
> disk I/O.

> If the arguments against raw-io are technically correct, why do we
> still support partition level access to swap ?

I'm no expert on this, but my understanding is that swap needs to have
the entire swap area in contiguous sectors, which can't be guaranteed
for files, but is a given with partitions.

Indeed, as I understand it, on the ext2 file system, ANY file that's
over 16 Megs in size is of necessity split into several separate
fragments...

> Assuming there's little performance difference between swap
> partitions and swap files, swap files are a big win. No hardwired
> decision on how much swap is needed, just create a swap file in one
> of the filesystems. If you find it's too big or too small it can be
> resized easilly. (Unlike a partition).

> On the flip side, if swap partitions do give some advantages,
> surely the same justification can be made for supporting raw-io.

I don't believe it can, since the specific advantage gained by swap
partitions that I mentioned above is in general irrelevant in all
other contexts...

> Note: I'm not saying the two cases are identical, just that there
> are fairly major simillarities.

Unfortunately, swap also has some unique requirements which make the
difference...

> I appreciate that it's far easier to keep something out of the
> kernel in the first place than to remove it later. However in the
> case of swap partitions I'm confident they could be removed
> painlessly. Just turn the partition into an e2fs one and create one
> large file. (Worst case).

Unfortunately wouldn't work - you'd have to creat a whole bunch of
separate files, each just below the fragmentation limit, and each
residing in a separate "block group" (the kernel's name, not mine), as
otherwise, they'd be unusable...

Best wishes from Riley.


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