On 08/01/2016 07:07 PM, Shaun Tancheff wrote: > On Mon, Aug 1, 2016 at 4:41 AM, Christoph Hellwig <hch@xxxxxx> wrote: >> >> Can you please integrate this with Hannes series so that it uses >> his cache of the zone information? > > Adding Hannes and Damien to Cc. > > Christoph, > > I can make a patch the marshal Hannes' RB-Tree into to a block report, that is > quite simple. I can even have the open/close/reset zone commands update the > RB-Tree .. the non-private parts anyway. I would prefer to do this around the > CONFIG_SD_ZBC support, offering the existing type of patch for setups that do > not need the RB-Tree to function with zoned media. > > I do still have concerns with the approach which I have shared in smaller > forums but perhaps I have to bring them to this group. > > First is the memory consumption. This isn't really much of a concern for large > servers with few drives but I think the embedded NAS market will grumble as > well as the large data pods trying to stuff 300+ drives in a chassis. > > As of now the RB-Tree needs to hold ~30000 zones. > sizeof() reports struct blk_zone to use 120 bytes on x86_64. This yields > around 3.5 MB per zoned drive attached. > Which is fine if it is really needed, but most of it is fixed information > and it can be significantly condensed (I have proposed 8 bytes per zone held > in an array as more than adequate). Worse is that the crucial piece of > information, the current wp needed for scheduling the next write, is mostly > out of date because it is updated only after the write completes and zones > being actively written to must work off of the last location / size that was > submitted, not completed. The work around is for that tracking to be handled > in the private_data member. I am not saying that updating the wp on > completing a write isn’t important, I am saying that the bi_end_io hook is > the existing hook that works just fine. > Which _actually_ is not true; with my patches I'll update the write pointer prior to submit the I/O (on the reasoning that most of the time I/O will succeed) and re-read the zone information if an I/O failed. (Which I'll have to do anyway as after an I/O failure the write pointer status is not clearly defined.) I have thought about condensing the RB tree information, but then I figured that for 'real' SMR handling we cannot assume all zones are of fixed size, and hence we need all the information there. Any condensing method would assume a given structure of the zones, which the standard just doesn't provide. Or am I missing something here? As for write pointer handling: yes, the write pointer on the zones is not really useful for upper-level usage. Where we do need it is to detect I/O which is crossing the write pointer (eg when doing reads over the entire zone). As per spec you will be getting an I/O error here, so we need to split the I/O on the write pointer to get valid results back. > This all tails into domain responsibility. With the RB-Tree doing half of the > work and the ‘responsible’ domain handling the active path via private_data > why have the split at all? It seems to be a double work to have second object > tracking the first so that I/O scheduling can function. > Tracking the zone states via RB trees is mainly to handly host-managed drives seamlessly. Without an RB tree we will be subjected to I/O errors during boot, as eg with new drives we inevitably will try to read beyond the write pointer, which in turn will generate I/O errors on certain drives. I do agree that there is no strict need to setup an RB tree for host-aware drives; I can easily add an attribute/flag to disable RB trees for those. However, tracking zone information in the RB tree _dramatically_ speeds up device initialisation; issuing a blkdev_discard() for the entire drive will take _ages_ without it. > Finally is the error handling path when the RB-Tree encounters and error it > attempts to requery the drive topology virtually guaranteeing that the > private_data is now out-of-sync with the RB-Tree. Again this is something > that can be better encapsulated in the bi_end_io to be informed of the > failed I/O and schedule the appropriate recovery (including re-querying the > zone information of the affected zone(s)). > Well, if we have an RB tree with write pointers of course we need to re-read the zone information on failure (and I thought I did that?). Plus the error information will be propagated via the usual mechanism, so they need to take care of updating any information in ->private_data. I'm perfectly fine with integrating your patches for the various blkdev_XX callouts and associated ioctls; Jens favours that approach, too, so we should be combining those efforts. Cheers, Hannes -- Dr. Hannes Reinecke Teamlead Storage & Networking hare@xxxxxxx +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton HRB 21284 (AG Nürnberg) -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
