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Re: [PATCH 4/5] Fix loop test

From: Jim Meyering
Subject: Re: [PATCH 4/5] Fix loop test
Date: Fri, 23 Dec 2011 22:05:26 +0100

Phillip Susi wrote:
> On 12/22/2011 04:54 PM, Jim Meyering wrote:
>> I'd really like to have a way to run two or more scsi_debug-using
>> tests in parallel but that is not possible: the kernel allows only one.
>> Note that for many of our scsi-debug-using tests, you cannot duplicate
>> the functionality using a loop device.  Besides, even where you can,
> I thought the only reason you needed scsi_debug was because loop
> devices couldn't be partitioned.


> What other reason is there?

If you take a look, I'll bet you can guess:
(read about scsi_debug in the kernel docs for details,
 or for a quick answer, read my prior reply that you quoted, below)

  $ git grep -h scsi_debug_setup_ tests
  scsi_debug_setup_ sector_size=$ss dev_size_mb=90 > dev-name ||
  scsi_debug_setup_ dev_size_mb=80 > dev-name ||
  scsi_debug_setup_ dev_size_mb=1 > dev-name ||
  scsi_debug_setup_ dev_size_mb=550 > dev-name ||
  scsi_debug_setup_ dev_size_mb=10 > dev-name ||
  scsi_debug_setup_ dev_size_mb=8 sector_size=4096 > dev-name ||
  scsi_debug_setup_ physblk_exp=3 lowest_aligned=7 num_parts=4 > dev-name ||
  scsi_debug_setup_ physblk_exp=3 lowest_aligned=7 sector_size=$ss > dev-name ||
  scsi_debug_setup_ sector_size=$ss dev_size_mb=$n_MiB > dev-name ||
  scsi_debug_setup_ sector_size=$ss dev_size_mb=$n_MiB > dev-name ||

>> because we don't use e.g., special alignment or block size, you risk
>> being unable to create a loop device due to inherent limitations, or,
>> if you create a few because their tests are running in parallel, you
>> risk perturbing regular processes that (albeit rarely) need to create a
>> loop device.  That is part of the reason for my using a private-homed
>> device tree: there I know I'm starting from a clean slate: i.e., far
>> less risk of interfering with some other loop-creating process.
> I don't follow.  How does using loop devices interfere with other
> processes doing the same?  And how does using your own /dev directory
> help at all?  Just because you have your own /dev directory doesn't
> mean you have your own set of devices; any devices that actually exist
> when you start still exist ( and are in use ) so you can't re-create a
> private copy, and any devices you create still show up in the real
> /dev.

I guess the isolation was useful only for lvm-related things,
not for loop-related.

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