Re: [dm-devel] [PATCH 0/2] block: blk_interposer v3

From: Sergei Shtepa <sergei.shtepa@veeam.com>
To: Bart Van Assche <bvanassche@acm.org>
Cc: "snitzer@redhat.com" <snitzer@redhat.com>,
	Pavel Tide <Pavel.TIde@veeam.com>,
	"ming.lei@redhat.com" <ming.lei@redhat.com>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"dm-devel@redhat.com" <dm-devel@redhat.com>,
	"agk@redhat.com" <agk@redhat.com>
Subject: Re: [dm-devel] [PATCH 0/2] block: blk_interposer v3
Date: Mon, 1 Feb 2021 21:18:35 +0300	[thread overview]
Message-ID: <20210201181835.GA2515@veeam.com> (raw)
In-Reply-To: <5c6c936a-f213-eaa3-f4fb-3461a0b4dbe1@acm.org>

The 02/01/2021 18:45, Bart Van Assche wrote:
> On 1/28/21 9:12 AM, Sergei Shtepa wrote:
> > I`m ready to suggest the blk_interposer again.
> > blk_interposer allows to intercept bio requests, remap bio to
> > another devices or add new bios.
> > 
> > This version has support from device mapper.
> > 
> > For the dm-linear device creation command, the `noexcl` parameter
> > has been added, which allows to open block devices without
> > FMODE_EXCL mode. It allows to create dm-linear device on a block
> > device with an already mounted file system.
> > The new ioctl DM_DEV_REMAP allows to enable and disable bio
> > interception.
> > 
> > Thus, it is possible to add the dm-device to the block layer stack
> > without reconfiguring and rebooting.
> 
> What functionality does this driver provide that is not yet available in 
> a RAID level 1 (mirroring) driver + a custom dm driver? My understanding 
> is that there are already two RAID level 1 drivers in the kernel tree 
> and that both driver support sending bio's to two different block devices.
> 
> Thanks,
> 
> Bart.

Hi Bart.

The proposed patch is not realy aimed at RAID1.

Creating a new dm device in the non-FMODE_EXCL mode and then remaping bio
requests from the regular block device to the new DM device using
the blk_interposer will allow to use device mapper for regular devices.
For dm-linear, there is not much benefit from using blk_interposer.
This is a good and illustrative example. Later, using blk-interposer,
it will be possible to connect the dm-cache "on the fly" without having
to reboot and/or reconfigure.
My intention is to let users use dm-snap to create snapshots of any device.
blk-interposer will allow to add new features to Device Mapper.

As per Daniel's advice I want to add a documentation, I'm working on it now.
The documentation will also contain a description of new features that
blk_interposer will add to Device Mapper

Thanks.

-- 
Sergei Shtepa
Veeam Software developer.

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