From: Damien Le Moal <Damien.LeMoal@wdc.com>
To: Hannes Reinecke <hare@suse.de>,
"Martin K. Petersen" <martin.petersen@oracle.com>
Cc: "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
Jens Axboe <axboe@kernel.dk>,
"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
"linux-ide@vger.kernel.org" <linux-ide@vger.kernel.org>
Subject: Re: [PATCH v3 0/4] Initial support for multi-actuator HDDs
Date: Fri, 6 Aug 2021 08:52:36 +0000 [thread overview]
Message-ID: <DM6PR04MB7081A6623985C0F299A8AB21E7F39@DM6PR04MB7081.namprd04.prod.outlook.com> (raw)
In-Reply-To: c3a28f3f-52c4-e7bc-8bd7-bec2feae25fa@suse.de
On 2021/08/06 17:36, Hannes Reinecke wrote:
> On 8/6/21 6:05 AM, Damien Le Moal wrote:
>> On 2021/08/06 12:42, Martin K. Petersen wrote:
>>>
>>> Damien,
>>>
>>>> Single LUN multi-actuator hard-disks are cappable to seek and execute
>>>> multiple commands in parallel. This capability is exposed to the host
>>>> using the Concurrent Positioning Ranges VPD page (SCSI) and Log (ATA).
>>>> Each positioning range describes the contiguous set of LBAs that an
>>>> actuator serves.
>>>
>>> I have to say that I prefer the multi-LUN model.
>>
>> It is certainly easier: nothing to do :)
>> SATA, as usual, makes things harder...
>>
>>>
>>>> The first patch adds the block layer plumbing to expose concurrent
>>>> sector ranges of the device through sysfs as a sub-directory of the
>>>> device sysfs queue directory.
>>>
>>> So how do you envision this range reporting should work when putting
>>> DM/MD on top of a multi-actuator disk?
>>
>> The ranges are attached to the device request queue. So the DM/MD target driver
>> can use that information from the underlying devices for whatever possible
>> optimization. For the logical device exposed by the target driver, the ranges
>> are not limits so they are not inherited. As is, right now, DM target devices
>> will not show any range information for the logical devices they create, even if
>> the underlying devices have multiple ranges.
>>
>> The DM/MD target driver is free to set any range information pertinent to the
>> target. E.g. dm-liear could set the range information corresponding to sector
>> chunks from different devices used to build the dm-linear device.
>>
> And indeed, that would be the easiest consumer.
> One 'just' needs to have a simple script converting the sysfs ranges
> into the corresponding dm-linear table definitions, and create one DM
> device for each range.
> That would simulate the multi-LUN approach.
> Not sure if that would warrant a 'real' DM target, seeing that it's
> fully scriptable.
>
>>> And even without multi-actuator drives, how would you express concurrent
>>> ranges on a DM/MD device sitting on top of a several single-actuator
>>> devices?
>>
>> Similar comment as above: it is up to the DM/MD target driver to decide if range
>> information can be useful. For dm-linear, there are obvious cases where it is.
>> Ex: 2 single actuator drives concatenated together can generate 2 ranges
>> similarly to a real split-actuator disk. Expressing the chunks of a dm-linear
>> setup as ranges may not always be possible though, that is, if we keep the
>> assumption that a range is independent from others in terms of command
>> execution. Ex: a dm-linear setup that shuffles a drive LBA mapping (high to low
>> and low to high) has no business showing sector ranges.
>>
>>> While I appreciate that it is easy to just export what the hardware
>>> reports in sysfs, I also think we should consider how filesystems would
>>> use that information. And how things would work outside of the simple
>>> fs-on-top-of-multi-actuator-drive case.
>>
>> Without any change anywhere in existing code (kernel and applications using raw
>> disk accesses), things will just work as is. The multi/split actuator drive will
>> behave as a single actuator drive, even for commands spanning range boundaries.
>> Your guess on potential IOPS gains is as good as mine in this case. Performance
>> will totally depend on the workload but will not be worse than an equivalent
>> single actuator disk.
>>
>> FS block allocators can definitely use the range information to distribute
>> writes among actuators. For reads, well, gains will depend on the workload,
>> obviously, but optimizations at the block IO scheduler level can improve things
>> too, especially if the drive is being used at a QD beyond its capability (that
>> is, requests are accumulated in the IO scheduler).
>>
>> Similar write optimization can be achieved by applications using block device
>> files directly. This series is intended for this case for now. FS and bloc IO
>> scheduler optimization can be added later.
>>
>>
> Rumours have it that Paolo Valente is working on adapting BFQ to utilize
> the range information for better actuator utilisation.
Paolo has a talk on this subject scheduled for SNIA SDC 2021.
https://storagedeveloper.org/events/sdc-2021/abstracts#hd-Walker
> And eventually one should modify filesystem utilities like xfs to adapt
> the metadata layout to multi-actuator drives.
>
> The _real_ fun starts once the HDD manufactures starts putting out
> multi-actuator SMR drives :-)
Well, that does not change things that much in the end. The same constraints
remain, and the sector ranges will be aligned to zones. So no added difficulty.
>
> Cheers,
>
> Hannes
>
--
Damien Le Moal
Western Digital Research
prev parent reply other threads:[~2021-08-06 8:52 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-26 1:38 [PATCH v3 0/4] Initial support for multi-actuator HDDs Damien Le Moal
2021-07-26 1:38 ` [PATCH v3 1/4] block: Add concurrent positioning ranges support Damien Le Moal
2021-07-26 7:33 ` Hannes Reinecke
2021-07-26 8:30 ` Damien Le Moal
2021-07-26 8:47 ` Hannes Reinecke
2021-07-26 11:33 ` Damien Le Moal
2021-07-27 14:07 ` Paolo Valente
2021-07-27 23:44 ` Damien Le Moal
2021-08-10 8:23 ` Christoph Hellwig
2021-08-10 11:03 ` Damien Le Moal
2021-08-10 16:02 ` hch
2021-08-10 23:46 ` Damien Le Moal
2021-07-26 1:38 ` [PATCH v3 2/4] scsi: sd: add " Damien Le Moal
2021-08-10 8:24 ` Christoph Hellwig
2021-07-26 1:38 ` [PATCH v3 3/4] libata: support concurrent positioning ranges log Damien Le Moal
2021-07-26 7:34 ` Hannes Reinecke
2021-08-10 8:26 ` Christoph Hellwig
2021-07-26 1:38 ` [PATCH v3 4/4] doc: document sysfs queue/cranges attributes Damien Le Moal
2021-07-26 7:35 ` Hannes Reinecke
2021-08-10 8:27 ` Christoph Hellwig
2021-08-10 11:04 ` Damien Le Moal
2021-07-28 22:59 ` [PATCH v3 0/4] Initial support for multi-actuator HDDs Damien Le Moal
2021-08-06 2:12 ` Damien Le Moal
2021-08-06 3:41 ` Martin K. Petersen
2021-08-06 4:05 ` Damien Le Moal
2021-08-06 8:35 ` Hannes Reinecke
2021-08-06 8:52 ` Damien Le Moal [this message]
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