linux-kernel - Re: [PATCH 0/6] detect online disk resize

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Date:	Wed, 24 Sep 2008 19:23:49 +0400
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Andrew Patterson <andrew.patterson@...com>
CC:	linux-scsi@...r.kernel.org, James.Bottomley@...senPartnership.com,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	axboe@...nel.dk, andmike@...ux.vnet.ibm.com, mike.miller@...com,
	genanr@...phone.com, jmoyer@...hat.com
Subject: Re: [PATCH 0/6] detect online disk resize


Andrew Patterson wrote:
> This is version 2 of a patch series I submitted back in May 2008. This
> version now checks for device size changes in the rescan_partitions()
> routine.  Which in turn is called when a device is opened and in the
> BLKRRPART ioctl.
> 
> I am resubmitting this patch series as I got little response the
> previous time.  Al Viro has told me offline that he would look at it this
> time.
> 
> This patch series handles online disk resizes that are currently not
> completely recognized by the kernel using the existing revalidate_disk
> routines.  An online resize can occur when growing or shrinking a
> Fibre Channel LUN or perhaps by adding a disk to an existing RAID
> volume.
> 
> The kernel currently recognizes a device size change when the
> lower-level revalidate_disk routines are called; however, the block
> layer does not use the new size while it has any current openers on
> the device. So, for example, if LVM has an volume open on the device,
> you will generally not see the size change until after a reboot. We
> fix this problem by creating a wrapper to be used with lower-level
> revalidate_disk routines.  This wrapper first calls the lower-level
> driver's revalidate_disk routine. It then compares the gendisk
> capacity to the block devices inode size. If there is a difference, we
> adjust the block device's size. If the size has changed, we then flush
> the disk for safety. The size is also checked in rescan_partitions
> which is called when the device is opened or when the BLKRRPART ioctl
> is called.
> 
> There are several ways to "kick off" a device size change:
> 
> 1. For SCSI devices do:
> 
>   # echo 1 > /sys/class/scsi_device/<device>/device/rescan
> 
>   or
> 
>   # blockdev --rereadpt <device file>
> 
> 2. Other devices (not device mapper)
> 
>   # blockdev --rereadpt <device file>

It would be great if you extend this functionality to automatic resizing 
by handling "CAPACITY DATA HAS CHANGED" Unit Attention. If necessary, I 
can assist in testing by adding online virtual disks resize feature 
together with generation of that UA in SCST (http://scst.sf.net). (It's 
not been implemented yet, because no known initiators so far have been 
able to use this feature.)

Vlad

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