[Lustre-devel] Wide striping
Nathan_Rutman at xyratex.com
Wed Oct 5 09:06:15 PDT 2011
On Oct 4, 2011, at 5:25 PM, wangdi wrote:
On 10/03/2011 01:15 PM, Nathan Rutman wrote:
Oracle BZ-4424 (continued in WC LU-80) adds support for larger OST stripe counts via increased EXT4 EA sizes.
Some problems with this are:
1) increased MDT storage and network loading for transmitting the object list
2) relative low new limit (1350 up from 160)
We have been thinking about a different wide-striping method that doesn't have these problems. The basic idea is to create a new stripe type that encodes the list of OSTs compactly, and then using the same (or a calculable) object identifier (or FID) on all these OSTs.
Our version of widestriping does not involve increasing the EA size at all, but instead utilizes a new stripe pattern. (This will not be understandable by older Lustre versions, which will generate an error locally, or potentially we can convert into the BZ-4424 form if the layout fits in that format). A bitmap will identify which OSTs hold a stripe of this file. The bitmap should probably fit into current ext4 EA size limit, giving us ~32k stripes.
Some OST’s may be down at file creation time, or new OSTs added later; hence there will likely be holes in the bitmap (but relatively few). Start index will still be used, but stripe order will be strictly round-robin (we will wrap around). In other words, the stripe sequence will always be in linear OST order, starting from start_index, maybe skipping some holes, wrapping around to start_index-1.
Widestripe objects do not need a special sequence number (fid_seq); the MDT knows the file was created as widestriped and marks it as such (LOV_PATTERN_BITMAP). There are two options for OST object identification: common object ID and FID-on-OST.
Actually, we also discussed to use real object (IAM or other index format) to store the stripe pattern, instead of using EA. Of course it would use more space, but it would give us the potential to explore the stripe pattern.
One of the main (the only?) benefits of our design (over current BZ4424 widestriping) is that it does not need any more space than the old MDT stripe pattern. No additional storage, no additional network traffic to transmit pattern.
Common Object ID
The MDT tracks a special range of OST object ID’s (“wide stripe objectid” = WSO) that are used on all OSTs. The MDT assigns the next available WSO to the file, and this objectid is used on all the OSTs. The OSTs must never use these objects for regular striped files. A special precreation group for these objects is probably necessary, as well as orphan cleanup (the MDT should purge "hole" objects that aren’t allocated from a particular OST). The MDT should track the last assigned WSO; this will be the starting point for new wide striped files after recovery. Objects cannot be migrated from one OST to another, since this would result in out-of-order access. Similarly, stripes can never be added to holes.
Use a mapping of the MDT FID to uniquely determine an OST object. The clients and MDT add in the OST number to the MDT FID (probably just reserve one sequence per OST). (This allows the objects to potentially migrate to different OSTs). The OSTs then internally must map the FID to a local object id. Note this allows OST-local precreation pools, getting the MDT out of the precreate/orphan cleanup business and potentially improving create speeds, and also facilitates "create on write" semantics. The FID can be assigned during the first access to OST object.
I am not sure I follow your idea here. You mean the OST needs internally map MDT FID(added in OST number) to object id (or inode ino) ?
So there are no real OST FID?
I suppose -- this is just a mapping of the MDT fid to the local OST object id, via a local lookup on the OST. There would be something like the OI to do this mapping.
But you also said "The FID can be assigned during the first access to OST object.", Could you please explain more here?
Since the FID -> Objid mapping is performed locally, it doesn't need to be assigned until the first write. This is not integral to the design, just a side effect.
The big problem here is that FID>OBJID ( or better FID->inode id ) translation is absent from the OSTs today. See http://wiki.lustre.org/images/e/e9/SC09-FID-on-OST.pdf <http://wiki.lustre.org/images/e/e9/SC09-FID-on-OST.pdf> (what is the current state of this?) There is also some work in this direction in the OST restructuring work (“Orion” WC branch, ORI-300(?), scheduled for Lustre 2.4).
There's a few questions here, probably the first of which is "is it worthwhile to spend effort on this, or is BZ4424 good enough?" Then there is the question of object identification, where FID-on-OST is more flexible, but also significantly more work (and risk). Also, I thought I understood from the EOFS Summit that WC also has a separate FID-on-OST project (separate from Orion that is) -- can someone tell me the state of that?
FID-on-OST is actually part of DNE(dirtribute name space) phase I. It basically follows current fid client server infrastructure.
1. MDT is the fid client, which requests fid from the OST and allocates fids for the object during pre-creation.
2. OST is the fid server, which will allocate the FIDs to MDTs and requests super fid sequence from fid control server (root MDT).
3. Similar as MDT FID, there will be OI to map FID to object inside OST.
To integrate with this, we would need to have a reserved sequence on each OST that the MDT could assign FIDs from --
the MDT would need to use the same Object ID on all OSTs. For DNE, there would need to be a reserved sequence per OST per MDT.
The code will be release with DNE sometime next year.
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