Message-ID: <alpine.LFD.2.00.1401081627020.6207@localhost.localdomain>
Date:	Wed, 8 Jan 2014 16:45:26 +0100 (CET)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	"Theodore Ts'o" <tytso@....edu>
cc:	linux-ext4@...r.kernel.org
Subject: Re: A proposal for making ext4's journal more SMR (and flash)
 friendly

On Wed, 8 Jan 2014, Theodore Ts'o wrote:

> Date: Wed, 8 Jan 2014 10:20:37 -0500
> From: Theodore Ts'o <tytso@....edu>
> To: Lukáš Czerner <lczerner@...hat.com>
> Cc: linux-ext4@...r.kernel.org
> Subject: Re: A proposal for making ext4's journal more SMR (and flash)
>     friendly
> 
> On Wed, Jan 08, 2014 at 12:43:35PM +0100, Lukáš Czerner wrote:
> >So it means that we would have to have bigger journal which is
> >multiple zones (or bands) of size long, right ? However I assume that
> >the optimal journal size in this case will be very much dependent
> >on the workload used - for example small file workload or other metadata
> >heavy workloads would need bigger journal. Could we possibly make journal
> >size variable ?
> 
> The journal size is already variable, e.g., "mke2fs -J size=512M".
> But yes, the optimal journal size will be highly variable.  

Yes, but I meant variable while the file system is mounted. With some
boundaries of course. But I guess we'll have to think about it once
we actually have some code done and hardware to test on.

I am just mentioning this because there is a possibility of this
being a problem and I would not want users to pick the right journal
size for every file system.

> 
> > > The journal is not truncated when the file system is unmounted, and so
> > > there is no difference between mounting a file system which has been
> > > cleanly unmounted or after a system crash.
> > 
> > I would maybe argue that clean unmount might be the right time for
> > checkpoint and resetting journal head back to the beginning because
> > I do not see it as a performance sensitive operation. This would in
> > turn help us on subsequent mount and run.
> 
> Yes, maybe.  It depends on how the the SMR drive handles random
> writes.  I suspect that most of the time, if the zones are close to
> the 256MB to 512MB rather than 32MB, the SMR drive is not going to
> rewrite the entire block just to handle a couple of random writes.  If
> we are doing an unmount, and so we don't care about performance for
> these random writes, if there is a way for us to hint to the SMR drive
> that no, really, it really should do a full zone rewrite, even if we
> are only updating a dozen blocks out of the 256MB zone, then sure,
> this might be a good thing to do.
> 
> But if the SMR drive takes these random metadata writes and writes
> them to some staging area, then it might not improve performance after
> we reboot and remount the file system --- indeed, depending on the
> location and nature of the staging area, it might make things worse.
> 
> I think we will need to do some experiments, and perhaps get some
> input from SMR drive vendors.  They probably won't be willing to
> release detailed design information without our being under NDA, but
> we can probably explain the design, and watch how their faces grin or
> twitch or scowl.  :-)
> 
> BTW, even if we do have NDA information from one vendor, it might not
> necessarily follow that other vendors use the same tradeoffs.  So even
> if some of us has NDA'ed information from one or two vendors, I'm a
> bit hesitant about hard-coding the design based on what they tell us.
> Besides the risk that one of the vendor might do things differently,
> there is also the concern that future versions of the drive might use
> different schemes for managing the logical->physical translation
> layer.  So we will probably want to keep our implementation and design
> flexible.

I very much agree, firmware implementation of those drives will
probably change a lot during first generations.

> 
> > While this helps a lot to avoid random writes it could possibly
> > result in much higher seek rates especially with bigger journals.
> > We're trying hard to keep data and associated metadata close
> > together and this would very much break that. This might be
> > especially bad with SMR devices because those are designed to be much
> > bigger in size. But of course this is a trade-off which makes it
> > very important to have good benchmark.
> 
> While the file system is mounted, if the metadata block is being
> referenced frequently, it will be in kept in memory, so the fact that
> it would have to seek to some random journal location if we need to
> read that metadata block might not be a big deal.  (This is similar to
> the argument used by log-structured file systems which claims that if
> we have enough memory, the fact that the metadata is badly fragmented
> doesn't matter.  Yes, if we are under heavy memory pressure, it might
> not work out.)
> 
> > I assume that the information about the newest commits for
> > particular metadata blocks would be kept in memory ? Otherwise it
> > would be quite expensive operation. But it seems unavoidable on
> > mount time, so it might really be better to clear the journal at
> > unmount when we should have all this information already in memory ?
> 
> Information about all commits and what blocks are still associated
> with them is alreaady being kept in memory.  Currently this is being
> done via a jh/bh; we'd want to do this differently, since we wouldn't
> necessarily enforce that all blocks which are in the journal must be
> in the buffer cache.  (Although if we did keep all blocks in the
> journal in the buffer cache, it would address the issue you raised
> above, at the expense of using a large amount of memory --- more
> memory than we would be comfortable using, although I'd bet is still
> less memory than, say, ZFS requires. :-)

I did not even think about always keeping those blocks in memory, it
should definitely be a subject to memory reclaim. With big enough
journal and the right workload this could grow out of proportions :)

> 
>        	      	    	      	      	     	- Ted
> 
> P.S.  One other benefit of this design which I forgot to mention in
> this version of the draft.  Using this scheme would also allow us to
> implement true read-only mounts and file system checks, without
> requiring that we modify the file system by replaying the journal
> before proceeding with the mount or e2fsck run.

Right, that would be nice side effect.

Thanks!
-Lukas

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