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From: Steven Davies <>
To: Anand Jain <>,,,
Subject: Re: [PATCH v7 rebased 0/5] readmirror feature (sysfs and in-memory only approach; with new read_policy device)
Date: Fri, 22 May 2020 20:15:32 +0100
Message-ID: <> (raw)
In-Reply-To: <>

On 19/05/2020 11:02, Anand Jain wrote:
> On 16/5/20 3:58 am, David Sterba wrote:
>> On Thu, Apr 30, 2020 at 05:02:27PM +0800, Anand Jain wrote:
>>>    I am not sure if this will be integrated in 5.8 and worth the time to
>>>    rebase. Kindly suggest.
>> The preparatory work is ok, but the actual mirror selection policy
>> addresses a usecase that I think is not the one most users are
>> interested in. Devices of vastly different performance capabilities like
>> rotational disks vs nvme vs ssd vs network block devices in one
>> filesystem are not something commonly found.
>> What we really need is a saner balancing mechanism than pid-based, that
>> is also going to be used any time there are more devices from the same
>> speed class for the fast devices too.

I can't find documentation about how mdadm chooses which mirror to read 
from (in the absence of --write-mostly) but everything suggests it uses 
different mirrors for different processes which sounds the same as or 
very much like %pid. Are you thinking along the lines of checking I/O 
queue length or average seek/read time when choosing the device?

> There are two things here, the read_policy framework in the preparatory
> patches and a new balancing or read_policy, device.
>> So, no the patchset is not on track for a merge without the improved
>> default balancing.
> It can be worked on top of the preparatory read_policy framework?
> This patchset does not change any default read_policy (or balancing)
> which is pid as of now. Working on a default read_policy/balancing
> was out of the scope of this patchset.
>> The preferred device for reads can be one of the
>> policies, I understand the usecase and have not problem with that
>> although wouldn't probably have use for it.
> For us, read_policy:device helps to reproduce raid1 data corruption
> And xfstests btrfs/14[0-3] can be improved so that the reads directly
> go the device of the choice, instead of waiting for the odd/even pid.
> Common configuration won't need this, advance configurations assembled
> with heterogeneous devices where read performance is more critical than
> write will find read_policy:device useful.

+1 for merging this as-is; I'm one of the users with heterogeneous 
devices (SATA SSD and NVMe SSD) and it would bring a feature similar to 
mdadm's --write-mostly. I've seen a few other requests for this feature 
on other discussion fora as well.

Steven Davies

      parent reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-06 11:51 Anand Jain
2020-04-06 11:51 ` [PATCH v7 1/5] btrfs: add btrfs_strmatch helper Anand Jain
2020-04-06 11:51 ` [PATCH v7 2/5] btrfs: create read policy framework Anand Jain
2020-04-06 11:51 ` [PATCH v7 3/5] btrfs: create read policy sysfs attribute, pid Anand Jain
2020-05-19 10:07   ` Johannes Thumshirn
2020-05-20  8:54     ` Anand Jain
2020-05-20  8:55       ` Johannes Thumshirn
2020-04-06 11:51 ` [PATCH v7 4/5] btrfs: introduce new device-state read_preferred Anand Jain
2020-04-06 11:51 ` [PATCH v7 5/5] btrfs: introduce new read_policy device Anand Jain
2020-04-30  9:02 ` [PATCH v7 rebased 0/5] readmirror feature (sysfs and in-memory only approach; with new read_policy device) Anand Jain
2020-05-15 19:58   ` David Sterba
2020-05-19 10:02     ` Anand Jain
2020-05-22 13:46       ` David Sterba
2020-05-26  7:23         ` Anand Jain
2020-05-22 19:15       ` Steven Davies [this message]

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