Re: Possible Idea with filesystem buffering.

Hans Reiser
2002-01-22T22:06:31Z

Shawn, I didn't respond to this because it seems like you are mixing in
issues relating to the elevator code into this, and so I don't really
understand you.

Hans

Shawn Starr wrote:

>Nobody wants to comment on this? :(
>
>Shawn.
>
>On Sun, 2002-01-20 at 21:29, Shawn Starr wrote:
>
>>On Mon, 21 Jan 2002, Anton Altaparmakov wrote:
>>
>>>[snip]
>>>At 00:57 21/01/02, Hans Reiser wrote:
>>>[snip]
>>> > Would be best if VM told us if we really must write that page.
>>>
>>>In theory the VM should never call writepage unless the page must be writen
>>>out...
>>>
>>>But I agree with you that it would be good to be able to distinguish the
>>>two cases. I have been thinking about this a bit in the context of NTFS TNG
>>>but I think that it would be better to have a generic solution rather than
>>>every fs does their own copy of the same thing. I envisage that there is a
>>>flush daemon which just walks around writing pages to disk in the
>>>background (there could be one per fs, or a generic one which fs register
>>>with, at their option they could have their own of course) in order to keep
>>>the number of dirty pages low and in order to minimize data loss on the
>>>event of system/power failure.
>>>
>>>This demon requires several interfaces though, with regards to journalling
>>>fs. The daemon should have an interface where the fs can say "commit pages
>>>in this list NOW and do not return before done", also a barrier operation
>>>would be required in journalling context. A transactions interface would be
>>>ideal, where the fs can submit whole transactions consisting of writing out
>>>a list of pages and optional write barriers; e.g. write journal pages x, y,
>>>z, barrier, write metadata, perhaps barrier, finally write data pages a, b,
>>>c. Simple file systems could just not bother at all and rely on the flush
>>>daemon calling the fs to write the pages.
>>>
>>>Obviously when this daemon writes pages the pages will continue being
>>>there. OTOH, if the VM calls write page because it needs to free memory
>>>then writepage must write and clean the page.
>>>
>>if they are dirty and written immediately to the disk they can be cleaned
>>from the queue. It would be nice if there was some way to have a checksum
>>verify the data was written back then wipe it from the queue.
>>
>>As an example: 5 operations requested, 2 already in queue.
>>
>>In queue) DIRTY write to disk (this task has been in the queue for a
>>while)
>>
>>In queue) not 'old' memory but must be written to disk
>>
>>pending queue:
>>
>>1) read operation
>>2) read operation
>>3) Write operation
>>4) write operation
>>
>>The daemon should resort the priority write dirty pages to disk then write
>>nay other pages that are left on queue, then get to read pages.
>>
>>
>>Notes:
>>
>>If there is only one operation in the queue (say write) and nothing else
>>comes along, then the daemon should force-write the data back to disk
>>after a period of timeout (the memory in the slot becomes dirty)
>>
>>If there's too many tasks in the queue and another one requires more
>>memory then whats left in the buffer/cache the daemon could request to
>>store the request in swap memory and put it in the queue, if the request
>>is a write request it would have more priority then any read requests
>>still and get completed quickly allowing for remaining queue events to
>>complete.
>>
>>Example:
>>
>>ReiserFS:
>> Operation A. Write (10K)
>> Operation B. Read (200K)
>> Operation C. Write (160K)
>>
>>
>>XFS:
>> Operation A. Read (63K)
>> Operation B. Read (3k)
>> Operation C. Write (10K)
>>
>>
>>EXT3:
>> Operation A. Write (290K)
>> Operation B. Write (90K)
>> Operation C. Read (3k)
>>
>>the kpagebuf (or whatever name). Would get all these requests and sort out
>>what needs to be done first as long as there's buffer/cache memory free
>>the write operations would be done as fast as possible, verified by some
>>checksum and purged from the queue, If there's no cache/buffer memory
>>free then all write queues reguardless of being in swap or cache/buffer need to be
>>written to disk.
>>
>>So:
>>kpagebuf queue (total available buffer/cache memory is say 512K)
>>
>> EXT3 Write (290K)
>> ReiserFS Write (160K)
>> ReiserFS Write (10K)
>> XFS Write (10K)
>> EXT3 Write (90K) - Goes in swap because total > 512K (Dirty x2 state)
>> ReiserFS Read (200K) - Swap (dirty x2)
>> XFS Read (63K) - Swap (dirty x2)
>> XFS Read (3K) - Swap (dirty x2)
>> EXT3 Read (3K) - Swap (dirty x2)
>>
>>* The daemon would check in order of filesystem registeration for whos
>>should be in the read queue first.
>>
>>* The daemon should maximize amount of memory stored in bufeer/cache to
>>try to prevent write requests having to go into swap.
>>
>>In the above queue, we have a lot of read operations and one write
>>operation in swap. Clean out the write operations since they are now dirty
>>(because there's no room for more operations in the buffer/cache). Move
>>the swapped write operation to the top of the queue and get rid of it.
>>Move the read operations from swap to queue since there is room again. **
>>NOTE ** because those read requests are now dirty they MUST be delt with
>>or they'll get stuck in the queue with more write requests overtaking
>>them.
>>
>>Maybe I've lost it but that's how I see it ;)
>>
>>Shawn.
>>
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