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RAID60 question

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  RAID60 question Greg <pld@sojka.co> - 2024-12-01 13:40 +0100
    Re: RAID60 question David Christensen <dpchrist@holgerdanske.com> - 2024-12-01 19:30 +0100
      Re: RAID60 question Greg <pld@sojka.co> - 2024-12-01 20:00 +0100
      Re: RAID60 question Jeffrey Walton <noloader@gmail.com> - 2024-12-02 09:10 +0100
        Re: RAID60 question David Christensen <dpchrist@holgerdanske.com> - 2024-12-03 01:40 +0100
    Re: RAID60 question Michael Paoli <michael.paoli@berkeley.edu> - 2024-12-02 04:50 +0100

#275094 — RAID60 question

Hi there,

I'm setting up MD-RAID0 on a top of HW-RAID6 devices (long story). I 
would like to confirm the following:

1. The RAID0 chunk size should be the stripe width of the
underlying RAID6 volumes.

2. The RAID0 metadata should be at the end of the device (metadata ver. 
1.0).

3. The stride and stripe-width of the ext4 fs should be set to the once 
used when creating RAID6 volumes.

Thanks in advance for any help
Greg

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#275106

On 12/1/24 04:27, Greg wrote:
> Hi there,
> 
> I'm setting up MD-RAID0 on a top of HW-RAID6 devices (long story). I 
> would like to confirm the following:
> 
> 1. The RAID0 chunk size should be the stripe width of the
> underlying RAID6 volumes.
> 
> 2. The RAID0 metadata should be at the end of the device (metadata ver. 
> 1.0).
> 
> 3. The stride and stripe-width of the ext4 fs should be set to the once 
> used when creating RAID6 volumes.
> 
> Thanks in advance for any help
> Greg


I have a SOHO network and I implemented file sharing many years ago. 
Around 2019, I switched from md to ZFS.  The learning curve has been 
non-trivial, but I am pleased with the results and expect ZFS will be a 
better solution going forward.


Regarding data migration in general, my previous approach had been 
in-place using one server and minimal disks (e.g. "cheap").  An operator 
error on a prior migration resulted in loss of some archival backups. 
So, I threw money at the problem this time around -- buy another server, 
buy more disks, build the new server, migrate the data, make the new 
server primary, rebuild the old server, and make the old server 
secondary.  And, backups before, during, and after.  So, more time and 
money, less risk, no data loss, and rebalanced data/ maximum performance 
afterwards.


David

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#275109

On 12/1/24 19:19, David Christensen wrote:
> On 12/1/24 04:27, Greg wrote:
>> Hi there,
>>
>> I'm setting up MD-RAID0 on a top of HW-RAID6 devices (long story). I 
>> would like to confirm the following:
>>
>> 1. The RAID0 chunk size should be the stripe width of the
>> underlying RAID6 volumes.
>>
>> 2. The RAID0 metadata should be at the end of the device (metadata 
>> ver. 1.0).
>>
>> 3. The stride and stripe-width of the ext4 fs should be set to the 
>> once used when creating RAID6 volumes.
>>
>> Thanks in advance for any help
>> Greg
> 
> 
> I have a SOHO network and I implemented file sharing many years ago. 
> Around 2019, I switched from md to ZFS.  The learning curve has been 
> non-trivial, but I am pleased with the results and expect ZFS will be a 
> better solution going forward.
> 
> 
> Regarding data migration in general, my previous approach had been 
> in-place using one server and minimal disks (e.g. "cheap").  An operator 
> error on a prior migration resulted in loss of some archival backups. 
> So, I threw money at the problem this time around -- buy another server, 
> buy more disks, build the new server, migrate the data, make the new 
> server primary, rebuild the old server, and make the old server 
> secondary.  And, backups before, during, and after.  So, more time and 
> money, less risk, no data loss, and rebalanced data/ maximum performance 
> afterwards.

I'm familiar with ZFS. Thanks for your suggestions but I have to stick 
to RAID60 (long story, like I said). I would be grateful for an answer 
to my original question.

Regards
Greg

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#275121

On Sun, Dec 1, 2024 at 3:47 PM David Christensen
<dpchrist@holgerdanske.com> wrote:
>
> On 12/1/24 04:27, Greg wrote:
> > Hi there,
> >
> > I'm setting up MD-RAID0 on a top of HW-RAID6 devices (long story). I
> > would like to confirm the following:
> >
> > 1. The RAID0 chunk size should be the stripe width of the
> > underlying RAID6 volumes.
> >
> > 2. The RAID0 metadata should be at the end of the device (metadata ver.
> > 1.0).
> >
> > 3. The stride and stripe-width of the ext4 fs should be set to the once
> > used when creating RAID6 volumes.
>
> I have a SOHO network and I implemented file sharing many years ago.
> Around 2019, I switched from md to ZFS.  The learning curve has been
> non-trivial, but I am pleased with the results and expect ZFS will be a
> better solution going forward.
>
>
> Regarding data migration in general, my previous approach had been
> in-place using one server and minimal disks (e.g. "cheap").  An operator
> error on a prior migration resulted in loss of some archival backups.
> So, I threw money at the problem this time around -- buy another server,
> buy more disks, build the new server, migrate the data, make the new
> server primary, rebuild the old server, and make the old server
> secondary.  And, backups before, during, and after.  So, more time and
> money, less risk, no data loss, and rebalanced data/ maximum performance
> afterwards.

???

What questions did you answer?

Jeff

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#275163

On 12/2/24 00:02, Jeffrey Walton wrote:
> On Sun, Dec 1, 2024 at 3:47 PM David Christensen
> <dpchrist@holgerdanske.com> wrote:
>>
>> On 12/1/24 04:27, Greg wrote:
>>> Hi there,
>>>
>>> I'm setting up MD-RAID0 on a top of HW-RAID6 devices (long story). I
>>> would like to confirm the following:
>>>
>>> 1. The RAID0 chunk size should be the stripe width of the
>>> underlying RAID6 volumes.
>>>
>>> 2. The RAID0 metadata should be at the end of the device (metadata ver.
>>> 1.0).
>>>
>>> 3. The stride and stripe-width of the ext4 fs should be set to the once
>>> used when creating RAID6 volumes.
>>
>> I have a SOHO network and I implemented file sharing many years ago.
>> Around 2019, I switched from md to ZFS.  The learning curve has been
>> non-trivial, but I am pleased with the results and expect ZFS will be a
>> better solution going forward.
>>
>>
>> Regarding data migration in general, my previous approach had been
>> in-place using one server and minimal disks (e.g. "cheap").  An operator
>> error on a prior migration resulted in loss of some archival backups.
>> So, I threw money at the problem this time around -- buy another server,
>> buy more disks, build the new server, migrate the data, make the new
>> server primary, rebuild the old server, and make the old server
>> secondary.  And, backups before, during, and after.  So, more time and
>> money, less risk, no data loss, and rebalanced data/ maximum performance
>> afterwards.
> 
> ???
> 
> What questions did you answer?
> 
> Jeff


I was providing anecdotal information for the OP and other readers to 
consider, because sometimes we lose sight of the forest for the trees.


David

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#275119

If you're not fully sure of any of those parameters, I'd suggest ...
do some testing.  E.g. if your target storage is quite large, maybe do only
2% of that, or possibly less - in total (approximate) size, but using the
presumed or being tested stripe/cluster/etc. sizes.  And test the performance,
and may want to use, e.g. sync option on mounted filesystems or the
like, to mostly
cancel out any host caching performance advantages, and get (at least closer to)
the actual drive (+RAID, etc.) performance, and how it would mostly behave on
cache misses (especially for writes, which will be your lowest performance
for RAID6/RAID60).  Can also inspect data on underlying devices (at least
as far down as one can go) with, e.g. od ... can even put marker patterns in
data to more easily identify exactly what data is landing where on the back-end
storage.  And as far as the md metadata, I believe it puts it at the
start, rather
than end.  In any case, I'd probably be inclined to go with the
default - likely less
confusing for anyone (e.g. even future you) to figure out exactly how
it's laid out,
if/when that becomes a question.  In any case, likewise, can test that, e.g.
scaled down, and examine the resultant data and where it lands.
And can use partitioning or losetup, etc. to limit the size of the target to
less than the full physical available, e.g. for testing.

And ... though not quite what you asked, device mapper, dmsetup, etc.
can be used to construct somewhat arbitrary layouts ... but that might
be even more confusing for anyone looking at it later.  Sometimes, however
that can be quite useful for special circumstances/requirements.  Thinking of
which, I not too long ago did that for demonstration purposes to help
someone out solving a data migration issue.  They essentially wanted to go from
quite large hardware RAID-5 to md raid5 - quite similar set of drives for each
(new ones for the md set).  Conceptually I basically thought layer RAID-1
atop that, sync, then break the mirrors.  That's not quite so easy as most any
(especially software) RAID-1 would typically want to write metadata on the same
devices - very undesirable in that case.  So, I did it with device
mapper using dmsetup,
essentially mirroring the underlying, while storing the metadata
external to those
devices.  Anyway, quite a bit more detail on that example run here:
https://lists.balug.org/mailman3/hyperkitty/list/balug-talk@lists.balug.org/message/CGZUVCF5WFM5I6GPKK5NW5DDK4OCMERK/

On Sun, Dec 1, 2024 at 4:36 AM Greg <pld@sojka.co> wrote:
>
> Hi there,
>
> I'm setting up MD-RAID0 on a top of HW-RAID6 devices (long story). I
> would like to confirm the following:
>
> 1. The RAID0 chunk size should be the stripe width of the
> underlying RAID6 volumes.
>
> 2. The RAID0 metadata should be at the end of the device (metadata ver.
> 1.0).
>
> 3. The stride and stripe-width of the ext4 fs should be set to the once
> used when creating RAID6 volumes.
>
> Thanks in advance for any help
> Greg

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