External bitmap, questions

External bitmap, questions

[Mathias Burén]

Hi,

From the mdadm manual:

"-b, --bitmap=
[...]
Note: external bitmaps are only known to work on ext2 and ext3.
Storing bitmap files on other filesystems may result in serious
problems."

I was planning to store the external bitmap on an ext4 partition. Will
this be a problem, or is the warning there because it hasn't been
tested enough but no problems found?

Thanks,
// Mathias

Re: External bitmap, questions

[NeilBrown]

On Sat, 19 Feb 2011 00:47:10 +0000 Mathias Burén <mathias.buren@gmail.com>
wrote:

> Hi,
> 
> >From the mdadm manual:
> 
> "-b, --bitmap=
> [...]
> Note: external bitmaps are only known to work on ext2 and ext3.
> Storing bitmap files on other filesystems may result in serious
> problems."
> 
> I was planning to store the external bitmap on an ext4 partition. Will
> this be a problem, or is the warning there because it hasn't been
> tested enough but no problems found?

External bitmaps use 'BMAP' to find where the file lives on the device and
then writes directly to the device - not through the filesystem.

So as long as there is no tail-packing to block migration happening it should
work fine.

I haven't looked inside ext4 but I am fairly confident that external bitmaps
will work properly.

NeilBrown

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Re: External bitmap, questions

[Mathias Burén]

On 19 February 2011 04:49, NeilBrown <neilb@suse.de> wrote:
> On Sat, 19 Feb 2011 00:47:10 +0000 Mathias Burén <mathias.buren@gmail.com>
> wrote:
>
>> Hi,
>>
>> >From the mdadm manual:
>>
>> "-b, --bitmap=
>> [...]
>> Note: external bitmaps are only known to work on ext2 and ext3.
>> Storing bitmap files on other filesystems may result in serious
>> problems."
>>
>> I was planning to store the external bitmap on an ext4 partition. Will
>> this be a problem, or is the warning there because it hasn't been
>> tested enough but no problems found?
>
> External bitmaps use 'BMAP' to find where the file lives on the device and
> then writes directly to the device - not through the filesystem.
>
> So as long as there is no tail-packing to block migration happening it should
> work fine.
>
> I haven't looked inside ext4 but I am fairly confident that external bitmaps
> will work properly.
>
> NeilBrown
>
>

Thanks, Neil. Are there any differences between using an internal or
external bitmap? Also, does one need a bitmap at all when not planning
to "mess" with the array? (does it provide any other purpose?)

// Mathias
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Re: External bitmap, questions

[NeilBrown]

On Sat, 19 Feb 2011 04:55:32 +0000 Mathias Burén <mathias.buren@gmail.com>
wrote:

> On 19 February 2011 04:49, NeilBrown <neilb@suse.de> wrote:
> > On Sat, 19 Feb 2011 00:47:10 +0000 Mathias Burén <mathias.buren@gmail.com>
> > wrote:
> >
> >> Hi,
> >>
> >> >From the mdadm manual:
> >>
> >> "-b, --bitmap=
> >> [...]
> >> Note: external bitmaps are only known to work on ext2 and ext3.
> >> Storing bitmap files on other filesystems may result in serious
> >> problems."
> >>
> >> I was planning to store the external bitmap on an ext4 partition. Will
> >> this be a problem, or is the warning there because it hasn't been
> >> tested enough but no problems found?
> >
> > External bitmaps use 'BMAP' to find where the file lives on the device and
> > then writes directly to the device - not through the filesystem.
> >
> > So as long as there is no tail-packing to block migration happening it should
> > work fine.
> >
> > I haven't looked inside ext4 but I am fairly confident that external bitmaps
> > will work properly.
> >
> > NeilBrown
> >
> >
> 
> Thanks, Neil. Are there any differences between using an internal or
> external bitmap? Also, does one need a bitmap at all when not planning
> to "mess" with the array? (does it provide any other purpose?)

The difference between internal and external is simply that one is stored in
the array - a copy on each device - and the other is stored externally to the
array  - a single copy in a file.
External are slightly harder to work with as you need somewhere separate to
store the bitmap (it cannot be in the array) and you need to tell mdadm where
to find it (so it needs to be listed in mdadm.conf).

I don't know what you mean by "mess" with the array.  The main purpose of the
bitmap is the accelerate resync after an unclean shutdown.  The secondary
purpose is to accelerate recovery if you remove and then re-add a device.
Maybe that is what you mean by "mess with".

So if you don't want to "mess" with the array and you are certain that your
machine will never crash, then you don't really need a bitmap.

NeilBrown

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Re: External bitmap, questions

[Mathias Burén]

On 19 February 2011 05:30, NeilBrown <neilb@suse.de> wrote:
> On Sat, 19 Feb 2011 04:55:32 +0000 Mathias Burén <mathias.buren@gmail.com>
> wrote:
>
>> On 19 February 2011 04:49, NeilBrown <neilb@suse.de> wrote:
>> > On Sat, 19 Feb 2011 00:47:10 +0000 Mathias Burén <mathias.buren@gmail.com>
>> > wrote:
>> >
>> >> Hi,
>> >>
>> >> >From the mdadm manual:
>> >>
>> >> "-b, --bitmap=
>> >> [...]
>> >> Note: external bitmaps are only known to work on ext2 and ext3.
>> >> Storing bitmap files on other filesystems may result in serious
>> >> problems."
>> >>
>> >> I was planning to store the external bitmap on an ext4 partition. Will
>> >> this be a problem, or is the warning there because it hasn't been
>> >> tested enough but no problems found?
>> >
>> > External bitmaps use 'BMAP' to find where the file lives on the device and
>> > then writes directly to the device - not through the filesystem.
>> >
>> > So as long as there is no tail-packing to block migration happening it should
>> > work fine.
>> >
>> > I haven't looked inside ext4 but I am fairly confident that external bitmaps
>> > will work properly.
>> >
>> > NeilBrown
>> >
>> >
>>
>> Thanks, Neil. Are there any differences between using an internal or
>> external bitmap? Also, does one need a bitmap at all when not planning
>> to "mess" with the array? (does it provide any other purpose?)
>
> The difference between internal and external is simply that one is stored in
> the array - a copy on each device - and the other is stored externally to the
> array  - a single copy in a file.
> External are slightly harder to work with as you need somewhere separate to
> store the bitmap (it cannot be in the array) and you need to tell mdadm where
> to find it (so it needs to be listed in mdadm.conf).
>
> I don't know what you mean by "mess" with the array.  The main purpose of the
> bitmap is the accelerate resync after an unclean shutdown.  The secondary
> purpose is to accelerate recovery if you remove and then re-add a device.
> Maybe that is what you mean by "mess with".
>
> So if you don't want to "mess" with the array and you are certain that your
> machine will never crash, then you don't really need a bitmap.
>
> NeilBrown
>
>

Hi,

Yes, you understood me correctly! Sorry, I should've been more clear.
That answers my questions, thanks a bunch :-)

// Mahtias
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Re: External bitmap, questions

[Roman Mamedov]

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On Sat, 19 Feb 2011 04:55:32 +0000
Mathias Burén <mathias.buren@gmail.com> wrote:

> > External bitmaps use 'BMAP' to find where the file lives on the device and
> > then writes directly to the device - not through the filesystem.
> >
> > So as long as there is no tail-packing to block migration happening it
> > should work fine.
> >
> > I haven't looked inside ext4 but I am fairly confident that external
> > bitmaps will work properly.
> >
> > NeilBrown
> >
> >
> 
> Thanks, Neil. Are there any differences between using an internal or
> external bitmap? Also, does one need a bitmap at all when not planning
> to "mess" with the array? (does it provide any other purpose?)

I have successfully stored an external bitmap on XFS for a brief time.
However it proved difficult to ensure that the device storing the bitmap is
mounted, accessible and writable prior to boot scripts attempting to run the md
arrays, so since then I just switched to an internal bitmap. The negative
performance it had can be minimized to almost zero by using a larger bitmap
chunk size.

-- 
With respect,
Roman

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Re: External bitmap, questions

[Mathias Burén]

On 19 February 2011 12:14, Roman Mamedov <rm@romanrm.ru> wrote:
> On Sat, 19 Feb 2011 04:55:32 +0000
> Mathias Burén <mathias.buren@gmail.com> wrote:
>
>> > External bitmaps use 'BMAP' to find where the file lives on the device and
>> > then writes directly to the device - not through the filesystem.
>> >
>> > So as long as there is no tail-packing to block migration happening it
>> > should work fine.
>> >
>> > I haven't looked inside ext4 but I am fairly confident that external
>> > bitmaps will work properly.
>> >
>> > NeilBrown
>> >
>> >
>>
>> Thanks, Neil. Are there any differences between using an internal or
>> external bitmap? Also, does one need a bitmap at all when not planning
>> to "mess" with the array? (does it provide any other purpose?)
>
> I have successfully stored an external bitmap on XFS for a brief time.
> However it proved difficult to ensure that the device storing the bitmap is
> mounted, accessible and writable prior to boot scripts attempting to run the md
> arrays, so since then I just switched to an internal bitmap. The negative
> performance it had can be minimized to almost zero by using a larger bitmap
> chunk size.
>
> --
> With respect,
> Roman
>

I ended up storing the bitmap on my root filesystem (ext4) for now.
It's currently at the default size but I think I'll recreate it with
64MB size.

// Mathias
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Re: External bitmap, questions

[Roman Mamedov]

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On Sat, 19 Feb 2011 12:20:41 +0000
Mathias Burén <mathias.buren@gmail.com> wrote:

> I ended up storing the bitmap on my root filesystem (ext4) for now.
> It's currently at the default size but I think I'll recreate it with
> 64MB size.

Thing is, the root filesystem is initially mounted read-only, after that
perhaps fsck is executed if needed, and only a bit later it is remounted
read-write. You need to check at which point in this process your distro's
boot scripts initialize md arrays, and whether or not you will end up trying
to do that with their bitmaps on a read-only FS.

-- 
With respect,
Roman

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