On Tue, 1 Jun 2010 13:31:23 +0530
Nitin Gupta <ngupta@xxxxxxxxxx> wrote:
> Currently, ramzwap devices (/dev/ramzswapX) can only
> be used as swap disks since it was hard-coded to consider
> only the first request in bio vector.
>
> Now, we iterate over all the segments in an incoming
> bio which allows us to handle all kinds of I/O requests.
>
> ramzswap devices can still handle PAGE_SIZE aligned and
> multiple of PAGE_SIZE sized I/O requests only. To ensure
> that we get always get such requests only, we set following
> request_queue attributes to PAGE_SIZE:
> - physical_block_size
> - logical_block_size
> - io_min
> - io_opt
>
> Note: physical and logical block sizes were already set
> equal to PAGE_SIZE and that seems to be sufficient to get
> PAGE_SIZE aligned I/O.
>
> Since we are no longer limited to handling swap requests
> only, the next few patches rename ramzswap to zram. So,
> the devices will then be called /dev/zram{0, 1, 2, ...}
Thanks for doing this - I think it's for the best..
> Usage/Examples:
> 1) Use as /tmp storage
> - mkfs.ext4 /dev/zram0
> - mount /dev/zram0 /tmp
hm, how does that work? The "device" will only handle page-sized and
page-aligned requests, won't it? Can you walk us through what happens
when the fs does a 512-byte I/O?
> - Double caching: We can potentially waste memory by having
> two copies of a page -- one in page cache (uncompress) and
> second in the device memory (compressed). However, during
> reclaim, clean page cache pages are quickly freed, so this
> does not seem to be a big problem.
Yes, clean pagecache is cheap. But what happens when the pagecache
copy of the page gets modified?
Or is it the case that once a compressed page gets copied out to
pagecache, the compressed version is never used again? If so, the
memory could be synchronously freed, so I guess I don't understand what
you mean here.
> - Stale data: Not all filesystems support issuing 'discard'
> requests to underlying block devices. So, if such filesystems
> are used over zram devices, we can accumulate lot of stale
> data in memory. Even for filesystems to do support discard
> (example, ext4), we need to see how effective it is.
Can you walk us through how zram uses discard requests?
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