Date:	Mon, 16 Mar 2015 15:06:27 -0400
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Adrian Palmer <adrian.palmer@...gate.com>
Cc:	Dave Chinner <david@...morbit.com>, xfs@....sgi.com,
	Linux Filesystem Development List 
	<linux-fsdevel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [ANNOUNCE] xfs: Supporting Host Aware SMR Drives

On Mon, 2015-03-16 at 12:23 -0600, Adrian Palmer wrote:
[...]
> >> == Data zones
> >>
> >> What we need is a mechanism for tracking the location of zones (i.e. start LBA),
> >> free space/write pointers within each zone, and some way of keeping track of
> >> that information across mounts. If we assign a real time bitmap/summary inode
> >> pair to each zone, we have a method of tracking free space in the zone. We can
> >> use the existing bitmap allocator with a small tweak (sequentially ascending,
> >> packed extent allocation only) to ensure that newly written blocks are allocated
> >> in a sane manner.
> >>
> >> We're going to need userspace to be able to see the contents of these inodes;
> >> read only access wil be needed to analyse the contents of the zone, so we're
> >> going to need a special directory to expose this information. It would be useful
> >> to have a ".zones" directory hanging off the root directory that contains all
> >> the zone allocation inodes so userspace can simply open them.
> >
> > The ZBC standard is being constructed.  However, all revisions agree
> > that the drive is perfectly capable of tracking the zone pointers (and
> > even the zone status).  Rather than having you duplicate the information
> > within the XFS metadata, surely it's better with us to come up with some
> > block way of reading it from the disk (and caching it for faster
> > access)?
> >
> 
> In discussions with Dr. Reinecke, it seems extremely prudent to have a
> kernel cache somewhere.  The SD driver would be the base for updating
> the cache, but it would need to be available to the allocators, the
> /sys fs for userspace utilities, and possibly other processes.  In
> EXT4, I don't think it's feasible to have the cache -- however, the
> metadata will MIRROR the cache ( BG# = Zone#, databitmap = WP, etc)

I think I've got two points: if we're caching it, we should have a
single cache and everyone should use it.  There may be a good reason why
we can't do this, but I'd like to see it explained before everyone goes
off and invents their own zone pointer cache.  If we do it in one place,
we can make the cache properly shrinkable (the information can be purged
under memory pressure and re-fetched if requested).

> >
> >> == Quantification of Random Write Zone Capacity
> >>
> >> A basic guideline is that for 4k blocks and zones of 256MB, we'll need 8kB of
> >> bitmap space and two inodes, so call it 10kB per 256MB zone. That's 40MB per TB
> >> for free space bitmaps. We'll want to suport at least 1 million inodes per TB,
> >> so that's another 512MB per TB, plus another 256MB per TB for directory
> >> structures. There's other bits and pieces of metadata as well (attribute space,
> >> internal freespace btrees, reverse map btrees, etc.
> >>
> >> So, at minimum we will probably need at least 2GB of random write space per TB
> >> of SMR zone data space. Plus a couple of GB for the journal if we want the easy
> >> option. For those drive vendors out there that are listening and want good
> >> performance, replace the CMR region with a SSD....
> >
> > This seems to be a place where standards work is still needed.  Right at
> > the moment for Host Managed, the physical layout of the drives makes it
> > reasonably simple to convert edge zones from SMR to CMR and vice versa
> > at the expense of changing capacity.  It really sounds like we need a
> > simple, programmatic way of doing this.  The question I'd have is: are
> > you happy with just telling manufacturers ahead of time how much CMR
> > space you need and hoping they comply, or should we push for a standards
> > way of flipping end zones to CMR?
> >
> 
> I agree this is an issue, but for HA (and less for HM), there is a lot
> of flexability needed for this.  In our BoFs at Vault, we talked about
> partitioning needs.  We cannot assume that there is 1 partition per
> disk, and that it has absolute boundaries.  Sure a data disk can have
> 1 partition from LBA 0 to end of disk, but an OS disk can't.  For
> example, GPT and EFI cause problems.  On the other end, gamers and
> hobbists tend to dual/triple boot....  There cannot be a onesize
> partition for all purposes.
> 
> The conversion between CMR and SMR zones is not simple.  That's a
> hardware format.  Any change in the LBA space would be non-linear.
> 
> One idea that I came up with in our BoFs is using flash with an FTL.
> If the manufacturers put in enough flash to cover 8 or so zones, then
> a command could be implemented to allow the flash to be assigned to
> zones.  That way, a limited number of CMR zones can be placed anywhere
> on the disk without disrupting format or LBA space.  However, ZAC/ZBC
> is to be applied to flash also...

Perhaps we need to step back a bit.  The problem is that most
filesystems will require some CMR space for metadata that is
continuously updated in place.  The amount will probably vary wildly by
specific filesystem and size, but it looks like everyone (except
possibly btrfs) will need some.  One possibility is that we let the
drives be reformatted in place, say as part of the initial filesystem
format, so the CMR requirements get tuned exactly.  The other is that we
simply let the manufacturers give us "enough" and try to determine what
"enough" is.

I suspect forcing a tuning command through the ZBC workgroup would be a
nice quick way of getting the manufacturers to focus on what is
possible, but I think we do need some way of closing out this either/or
debate (we tune or you tune).

> >
> >> === Crash recovery
> >>
> >> Write pointer location is undefined after power failure. It could be at an old
> >> location, the current location or anywhere in between. The only guarantee that
> >> we have is that if we flushed the cache (i.e. fsync'd a file) then they will at
> >> least be in a position at or past the location of the fsync.
> >>
> >> Hence before a filesystem runs journal recovery, all it's zone allocation write
> >> pointers need to be set to what the drive thinks they are, and all of the zone
> >> allocation beyond the write pointer need to be cleared. We could do this during
> >> log recovery in kernel, but that means we need full ZBC awareness in log
> >> recovery to iterate and query all the zones.
> >
> > If you just use a cached zone pointer provided by block, this should
> > never be a problem because you'd always know where the drive thought the
> > pointer was.
> 
> This would require a look at the order of updating the stack
> information, and also WCD vs WCE behavior.  As for the WP, the spec
> says that any data after the WP is returned with a clear pattern
> (zeros on Seagate drives) -- it is already cleared.

As long as the drive behaves to spec, our consistency algorithms should
be able to cope.  We would expect that on a crash the write pointer
would be further back than we think it should be, but then the FS will
just follow its consistency recovery procedures and either roll back or
forward the transactions from where the WP is at.  In some ways, the WP
will help us, because we do a lot of re-committing transactions that may
be on disk currently because we don't clearly know where the device
stopped writing data.

> >> === RAID on SMR....
> >>
> >> How does RAID work with SMR, and exactly what does that look like to
> >> the filesystem?
> >>
> >> How does libzbc work with RAID given it is implemented through the scsi ioctl
> >> interface?
> >
> > Probably need to cc dm-devel here.  However, I think we're all agreed
> > this is RAID across multiple devices, rather than within a single
> > device?  In which case we just need a way of ensuring identical zoning
> > on the raided devices and what you get is either a standard zone (for
> > mirror) or a larger zone (for hamming etc).
> >
> 
> I agree.  It's up to the DM to mangle the zones and provide proper
> modified zone info up to the FS.  In the case of mirror, keeps the
> same zone size, just half the total of zones (or half in a condition
> of read-only/full).  In stripped paradigms, double (or more if the
> zone sizes don't match, or if more that 2 drives) the zone size and
> let the DM mod the block numbers to determine the correct disk.  For
> EXT4, this REQUIRES the equivalent of 8k Blocks.
> 
> > James
> >
> >
> 
> == Kernel implementation
> 
> The allocator will need to learn about multiple allocation zones based on
> bitmaps. They aren't really allocation groups, but the initialisation and
> iteration of them is going to be similar to allocation groups. To get use going
> we can do some simple mapping between inode AG and data AZ mapping so that we
> keep some form of locality to related data (e.g. grouping of data by parent
> directory).
> 
> We can do simple things first - simply rotoring allocation across zones will get
> us moving very quickly, and then we can refine it once we have more than just a
> proof of concept prototype.
> 
> Optimising data allocation for SMR is going to be tricky, and I hope to be able
> to leave that to drive vendor engineers....

I think we'd all be interested in whether the write and return
allocation position suggested at LSF/MM would prove useful for this (and
whether the manufacturers are interested in prototyping it with us).

> Ideally, we won't need a zbc interface in the kernel, except to erase zones.
> I'd like to see an interface that doesn't even require that. For example, we
> issue a discard (TRIM) on an entire  zone and that erases it and
> resets the write
> pointer. This way we need no new infrastructure at the filesystem layer to
> implement SMR awareness. In effect, the kernel isn't even aware that it's an SMR
> drive underneath it.
> 
> 
> Dr. Reinecke has already done the Discard/TRIM stuff.  However, he's
> as of yet ignored the zone management pieces.  I have thought
> (briefly) of the possible need for a new allocator:  the group
> allocator.  As there can only be a few (relatively) zones available at
> any one time, We might need a mechanism to tell which are available
> and which are not.  The stack will have to collectively work together
> to find a way to request and use zones in an orderly fashion.

Here I think the sense of LSF/MM was that only allowing a fixed number
of zones to be open would get a bit unmanageable (unless the drive
silently manages it for us).  The idea of different sized zones is also
a complicating factor.  The other open question is that if we go for
fully drive managed, what sort of alignment, size, trim + anything else
should we do to make the drive's job easier.  I'm guessing we won't
really have a practical answer to any of these until we see how the
market responds.

James


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