Re: [PATCH] md/raid0: add config parameters to specify zone layout

From: Jason Baron
Date: Mon Apr 27 2020 - 17:11:30 EST




On 4/25/20 12:31 AM, Coly Li wrote:
> On 2020/3/26 23:28, Jason Baron wrote:
>> Let's add some CONFIG_* options to directly configure the raid0 layout
>> if you know in advance how your raid0 array was created. This can be
>> simpler than having to manage module or kernel command-line parameters.
>>
>
> Hi Jason,
>
> If the people who compiling the kernel is not the end users, the
> communication gap has potential risk to make users to use a different
> layout for existing raid0 array after a kernel upgrade.
>
> If this patch goes into upstream, it is very probably such risky
> situation may happen.
>
> The purpose of adding default_layout is to let *end user* to be aware of
> they layout when they use difference sizes component disks to assemble
> the raid0 array, and make decision which layout algorithm should be
> used. Such situation cannot be decided in kernel compiling time.

I agree that in general it may not be known at compile time. Thus,
I've left the default as RAID0_LAYOUT_NONE. However, there are
use-cases where it is known at compile-time which layout is needed.
In our use-case, we knew that we didn't have any pre-3.14 raid0
arrays. Thus, we can safely set RAID0_ALT_MULTIZONE_LAYOUT. So
this is a simpler configuration for us than setting module or command
line parameters.

Thanks,

-Jason

>
>> If the raid0 array was created by a pre-3.14 kernel, use
>> RAID0_ORIG_LAYOUT. If the raid0 array was created by a 3.14 or newer
>> kernel then select RAID0_ALT_MULTIZONE_LAYOUT. Otherwise, the default
>> setting is RAID0_LAYOUT_NONE, in which case the current behavior of
>> needing to specify a module parameter raid0.default_layout=1|2 is
>> preserved.
>>
>
> The difficulty is for a given md raid0 array, there is no clue whether
> it is built on pre-3.14 kernel or not. If the kernel is not configured
> and built by end user, we have risk that wrong algorithm will be applied
> to unmatched layout.
>> Thanks.
>
> Coly Li
>
>
>> Cc: Guoqing Jiang <guoqing.jiang@xxxxxxxxxxxxxxx>
>> Cc: NeilBrown <neilb@xxxxxxx>
>> Cc: Song Liu <songliubraving@xxxxxx>
>> Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>
>> ---
>> drivers/md/Kconfig | 55 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>> drivers/md/raid0.c | 7 +++++++
>> 2 files changed, 62 insertions(+)
>>
>> diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
>> index d6d5ab2..c0c6d82 100644
>> --- a/drivers/md/Kconfig
>> +++ b/drivers/md/Kconfig
>> @@ -79,6 +79,61 @@ config MD_RAID0
>>
>> If unsure, say Y.
>>
>> +choice
>> + prompt "RAID0 Layout"
>> + default RAID0_LAYOUT_NONE
>> + depends on MD_RAID0
>> + help
>> + A change was made in Linux 3.14 that unintentinally changed the
>> + the layout for RAID0. This can result in data corruption if a pre-3.14
>> + and a 3.14 or later kernel both wrote to the array. However, if the
>> + devices in the array are all of the same size then the layout would
>> + have been unaffected by this change, and there is no risk of data
>> + corruption from this issue.
>> +
>> + Unfortunately, the layout can not be determined by the kernel. If the
>> + array has only been written to by a 3.14 or later kernel its safe to
>> + set RAID0_ALT_MULTIZONE_LAYOUT. If its only been written to by a
>> + pre-3.14 kernel its safe to select RAID0_ORIG_LAYOUT. If its been
>> + written by both then select RAID0_LAYOUT_NONE, which will not
>> + configure the array. The array can then be examined for corruption.
>> +
>> + For new arrays you may choose either layout version. Neither version
>> + is inherently better than the other.
>> +
>> + Alternatively, these parameters can also be specified via the module
>> + parameter raid0.default_layout=<N>. N=2 selects the 'new' or multizone
>> + layout, while N=1 selects the 'old' layout or original layout. If
>> + unset the array will not be configured.
>> +
>> + The layout can also be written directly to the raid0 array via the
>> + mdadm command, which can be auto-detected by the kernel. See:
>> + <https://www.kernel.org/doc/html/latest/admin-guide/md.html#multi-zone-raid0-layout-migration>
>> +
>> +config RAID0_ORIG_LAYOUT
>> + bool "raid0 layout for arrays only written to by a pre-3.14 kernel"
>> + help
>> + If the raid0 array was only created and written to by a pre-3.14 kernel.
>> +
>> +config RAID0_ALT_MULTIZONE_LAYOUT
>> + bool "raid0 layout for arrays only written to be a 3.14 or newer kernel"
>> + help
>> + If the raid0 array was only created and written to by a 3.14 or later
>> + kernel.
>> +
>> +config RAID0_LAYOUT_NONE
>> + bool "raid0 layout must be specified via a module parameter"
>> + help
>> + If a raid0 array was written to by both a pre-3.14 and a 3.14 or
>> + later kernel, you may have data corruption. This option will not
>> + auto configure the array and thus you can examine the array offline
>> + to determine the best way to proceed. With RAID0_LAYOUT_NONE
>> + set, the choice for raid0 layout can be set via a module parameter
>> + raid0.default_layout=<N>. Or the layout can be written directly
>> + to the raid0 array via the mdadm command.
>> +
>> +endchoice
>> +
>> config MD_RAID1
>> tristate "RAID-1 (mirroring) mode"
>> depends on BLK_DEV_MD
>> diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
>> index 322386f..576eaa6 100644
>> --- a/drivers/md/raid0.c
>> +++ b/drivers/md/raid0.c
>> @@ -19,7 +19,14 @@
>> #include "raid0.h"
>> #include "raid5.h"
>>
>> +#if defined(CONFIG_RAID0_ORIG_LAYOUT)
>> +static int default_layout = RAID0_ORIG_LAYOUT;
>> +#elif defined(CONFIG_RAID0_ALT_MULTIZONE_LAYOUT)
>> +static int default_layout = RAID0_ALT_MULTIZONE_LAYOUT;
>> +#else
>> static int default_layout = 0;
>> +#endif
>> +
>> module_param(default_layout, int, 0644);
>>
>> #define UNSUPPORTED_MDDEV_FLAGS \
>>
>