On Tue, Aug 08, 2017 at 02:31:54PM -0600, Jens Axboe wrote:
> On 08/07/2017 10:36 AM, Shaohua Li wrote:
> > On Mon, Aug 07, 2017 at 10:29:05AM +0200, Hannes Reinecke wrote:
> >> On 08/05/2017 05:51 PM, Shaohua Li wrote:
> >>> From: Shaohua Li <s...@fb.com>
> >>>
> >>> In testing software RAID, I usually found it's hard to cover specific 
> >>> cases.
> >>> RAID is supposed to work even disk is in semi good state, for example, 
> >>> some
> >>> sectors are broken. Since we can't control the behavior of hardware, it's
> >>> difficult to create test suites to do destructive tests. But we can 
> >>> control the
> >>> behavior of software, software based disk is an obvious choice for such 
> >>> tests.
> >>> While we already have several software based disks for testing (eg, 
> >>> null_blk,
> >>> scsi_debug), none is for destructive testing, this is the reason we 
> >>> create a
> >>> new test block device.
> >>>
> >>> Currently the driver can create disk with following features:
> >>> - Bandwidth control. A raid array consists of several disks. The disks 
> >>> could
> >>>   run in different speed, for example, one disk is SSD and the other is 
> >>> HD.
> >>>   Actually raid1 has a feature called write behind just for this. To test 
> >>> such
> >>>   raid1 feature, we'd like the disks speed could be controlled.
> >>> - Emulate disk cache. Software must flush disk cache to guarantee data is
> >>>   safely stored in media after a power failure. To verify if software 
> >>> works
> >>>   well, we can't simply use physical disk, because even software doesn't 
> >>> flush
> >>>   cache, the hardware probably will flush the cache. With a software
> >>>   implementation of disk cache, we can fully control how we flush disk 
> >>> cache in a
> >>>   power failure.
> >>> - Badblock. If only part of a disk is broken, software raid continues 
> >>> working.
> >>>   To test if software raid works well, disks must include some broken 
> >>> parts or
> >>>   bad blocks. Bad blocks can be easily implemented in software.
> >>>
> >>> While this is inspired by software raid testing, the driver is very 
> >>> flexible
> >>> for extension. We can easily add new features into the driver. The 
> >>> interface is
> >>> configfs, which can be configured with a shell script. There is a 
> >>> 'features'
> >>> attribute exposing all supported features. By checking this, we don't 
> >>> need to
> >>> worry about compability issues. For configuration details, please check 
> >>> the
> >>> first patch.
> >>>
> >> Any particular reason why you can't fold these changes into brd or 
> >> null_blk?
> >> After all, without those testing features it is 'just' another ramdisk
> >> driver...
> > 
> > null_blk isn't a good fit. ramdisk might be, but I try to not. We are adding
> > new interfaces, locks and so on. Adding the features into ramdisk driver 
> > will
> > mess it up. Binding it to ramdisk driver will make adding new features 
> > harder
> > too, because the test driver doesn't really care about performance while
> > ramdisk does.
> 
> I'm curious why null_blk isn't a good fit? You'd just need to add RAM
> storage to it. That would just be a separate option that should be set,
> ram_backing=1 or something like that. That would make it less critical
> than using the RAM disk driver as well, since only people that want a "real"
> data backing would enable it.
> 
> It's not that I'm extremely opposed to adding a(nother) test block driver,
> but we at least need some sort of reasoning behind why, which isn't just
> "not a good fit".

Ah, I thought the 'null' of null_blk means we do nothing for the disks. Of
course we can rename it, which means this point less meaningful. I think the
main reason is the interface. We will configure the disks with different
parameters and do power on/off for each disks (which is the key we can emulate
disk cache and power loss). The module paramter interface of null_blk doesn't
work for the usage. Of course, these issues can be fixed, for example, we can
make null_blk use the configfs interface. If you really prefer a single driver
for all test purpose, I can move the test_blk functionalities to null_blk.

Thanks,
Shaohua

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