Re: [CentOS] xfs not getting it right?

[Chris Adams]

Once upon a time, hw  said:
> Are you saying there is no difference between a RAID1 and a non-raid
> device as far as xfs is concerned?

Yes.

> What if you use hardware RAID?

No difference - same result.

> When you look at [1], it tells you to specify su and sw with hardware
> RAID and says it detects everything automatically with md-RAID.  It doesn´t
> have an example with RAID1 but one with RAID10 --- however, why would that
> make a difference?

Because RAID level 1 and RAID level 10 are different.  I suggest you
read:

https://en.wikipedia.org/wiki/RAID#Standard_levels

What is called "RAID 10" is really a combination of level 1 and level 0
(which one is higher/lower varies between implementations).

> If you read from both
> disks in a RAID1 simultaneously, you have to wait out the latency of both
> disks before you get the data at full speed, and it might be better to use
> stripes with them as well and read multiple parts of the data at the same
> time.

RAID level 1 has the same data on both drives.  You wouldn't be reading
the same data from both drives at the same time; reads would be spread
between the drives (I know the Linux software RAID tries to keep read
load fairly balanced between drives, I assume most hardware RAID
implementations do the same).

> Oh, ok.  How do you know what stripe size was picked by mdadm?  It seemd a
> good idea to go with defaults as far as possible.

Again, RAID level 1 has no stripes.
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Re: [CentOS] xfs not getting it right?

[Fred Smith]

On Wed, Sep 20, 2017 at 06:12:39PM +0200, hw wrote:
> Chris Adams wrote:
> >Once upon a time, hw  said:
> >>xfs is supposed to detect the layout of a md-RAID devices when creating the
> >>file system, but it doesn´t seem to do that:
> >>
> >>
> >># cat /proc/mdstat
> >>Personalities : [raid1]
> >>md10 : active raid1 sde[1] sdd[0]
> >>  499976512 blocks super 1.2 [2/2] [UU]
> >>  bitmap: 0/4 pages [0KB], 65536KB chunk
> >
> >RAID 1 has no "layout" (for RAID, that usually refers to striping in
> >RAID levels 0/5/6), so there's nothing for a filesystem to detect or
> >optimize for.
> 
> Are you saying there is no difference between a RAID1 and a non-raid
> device as far as xfs is concerned?

RAID1 is simply two or more drives with the same stuff on each drive.
its a simple form of redundancy. striping would put some of it on one
drive and more on another, and so forth for as many drives as you use
with some kind of redundancy or checksumming. RAID1 is much simpler:
simply make all the drives carry the same data.


> 
> What if you use hardware RAID?
> 
> When you look at [1], it tells you to specify su and sw with hardware
> RAID and says it detects everything automatically with md-RAID.  It doesn´t
> have an example with RAID1 but one with RAID10 --- however, why would that
> make a difference?  Aren´t there stripes in a RAID1?  If you read from both
> disks in a RAID1 simultaneously, you have to wait out the latency of both
> disks before you get the data at full speed, and it might be better to use

RAID1 wouldn't have that problem, necessarily. since all drives carry
the same data, it is necessary to read from only one of them. it is during
a write  operation that all drives are written, and even then they may not
be written at exactly the same time... one can be written and the data for the
other in buffer-cache until the system gets a chance to write it.

I don't know details, per the above, for Linux software raid, but I also
have a USB-attached HW raid box (jmicron chip) and I can watch the lights
on the drives and see it doing exactly that. I would find it hard to believe
that software raid in Linux is significantly different (in fact, i rather
suspect that the external raid box is probably running some older version
of Linux)
> stripes with them as well and read multiple parts of the data at the same
> time.
> 
> [1]: 
> http://xfs.org/index.php/XFS_FAQ#Q:_How_to_calculate_the_correct_sunit.2Cswidth_values_for_optimal_performance
> 
> 
> > The chunk size above is for the md-RAID write-intent bitmap; that's not
> > exposed information (for any RAID system that I'm aware of, software or
> > hardware) or something that filesystems can optimize for.
> 
> Oh, ok.  How do you know what stripe size was picked by mdadm?  It seemd a
> good idea to go with defaults as far as possible.
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-- 
 Fred Smith -- fre...@fcshome.stoneham.ma.us -
  "For him who is able to keep you from falling and to present you before his 
 glorious presence without fault and with great joy--to the only God our Savior
 be glory, majesty, power and authority, through Jesus Christ our Lord, before
 all ages, now and forevermore! Amen."
- Jude 1:24,25 (niv) -
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Re: [CentOS] xfs not getting it right?

[Jon Pruente]

On Wed, Sep 20, 2017 at 11:12 AM, hw  wrote:

> Are you saying there is no difference between a RAID1 and a non-raid
> device as far as xfs is concerned?
>
> What if you use hardware RAID?
>

RAID1 is mirroring. There is nothing to stripe because the virtual device
is almost identical to the physical drives, and they both see the same read
and write instructions in parallel.
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Re: [CentOS] xfs not getting it right?

[hw]

Chris Adams wrote:

Once upon a time, hw  said:

xfs is supposed to detect the layout of a md-RAID devices when creating the
file system, but it doesn´t seem to do that:


# cat /proc/mdstat
Personalities : [raid1]
md10 : active raid1 sde[1] sdd[0]
  499976512 blocks super 1.2 [2/2] [UU]
  bitmap: 0/4 pages [0KB], 65536KB chunk


RAID 1 has no "layout" (for RAID, that usually refers to striping in
RAID levels 0/5/6), so there's nothing for a filesystem to detect or
optimize for.


Are you saying there is no difference between a RAID1 and a non-raid
device as far as xfs is concerned?

What if you use hardware RAID?

When you look at [1], it tells you to specify su and sw with hardware
RAID and says it detects everything automatically with md-RAID.  It doesn´t
have an example with RAID1 but one with RAID10 --- however, why would that
make a difference?  Aren´t there stripes in a RAID1?  If you read from both
disks in a RAID1 simultaneously, you have to wait out the latency of both
disks before you get the data at full speed, and it might be better to use
stripes with them as well and read multiple parts of the data at the same
time.

[1]: 
http://xfs.org/index.php/XFS_FAQ#Q:_How_to_calculate_the_correct_sunit.2Cswidth_values_for_optimal_performance


> The chunk size above is for the md-RAID write-intent bitmap; that's not
> exposed information (for any RAID system that I'm aware of, software or
> hardware) or something that filesystems can optimize for.

Oh, ok.  How do you know what stripe size was picked by mdadm?  It seemd a
good idea to go with defaults as far as possible.
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Re: [CentOS] xfs not getting it right?

[Chris Adams]

Once upon a time, hw  said:
> I was expecting that the correct stripe size and stripe width would be used.

RAID level 1 has no stripe size or width.
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Re: [CentOS] xfs not getting it right?

[hw]

Stephen John Smoogen wrote:

On 20 September 2017 at 10:47, hw  wrote:


Hi,

xfs is supposed to detect the layout of a md-RAID devices when creating the
file system, but it doesn´t seem to do that:


# cat /proc/mdstat
Personalities : [raid1]
md10 : active raid1 sde[1] sdd[0]
  499976512 blocks super 1.2 [2/2] [UU]
  bitmap: 0/4 pages [0KB], 65536KB chunk


# mkfs.xfs /dev/md10p2
meta-data=/dev/md10p2isize=512agcount=4, agsize=30199892
blks
 =   sectsz=512   attr=2, projid32bit=1
 =   crc=1finobt=0, sparse=0
data =   bsize=4096   blocks=120799568, imaxpct=25
 =   sunit=0  swidth=0 blks
naming   =version 2  bsize=4096   ascii-ci=0 ftype=1
log  =internal log   bsize=4096   blocks=58984, version=2
 =   sectsz=512   sunit=0 blks, lazy-count=1
realtime =none   extsz=4096   blocks=0, rtextents=0


# mkfs.xfs -f -d su=64m,sw=2 /dev/md10p2
meta-data=/dev/md10p2isize=512agcount=16, agsize=7553024
blks
 =   sectsz=512   attr=2, projid32bit=1
 =   crc=1finobt=0, sparse=0
data =   bsize=4096   blocks=120799568, imaxpct=25
 =   sunit=16384  swidth=32768 blks
naming   =version 2  bsize=4096   ascii-ci=0 ftype=1
log  =internal log   bsize=4096   blocks=58984, version=2
 =   sectsz=512   sunit=8 blks, lazy-count=1
realtime =none   extsz=4096   blocks=0, rtextents=0


The 64MB chunk size was picked my mdadm automatically.  The device is made
from two disks, and xfs either doesn´t figure that out, or it decided to
ignore the layout of the underlying RAID.

Am I doing something wrong here, or is xfs in Centos somehow different?
Do, or must, we always specify the apporpriate values for su and sw or
did xfs ignore them because what it picked is better?



I don't know enough to answer, but I do have a question.. what were
you expecting xfs to do (and what filesystems do that?) Thanks


I was expecting that the correct stripe size and stripe width would be used.
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Re: [CentOS] xfs not getting it right?

[Chris Adams]

Once upon a time, hw  said:
> xfs is supposed to detect the layout of a md-RAID devices when creating the
> file system, but it doesn´t seem to do that:
> 
> 
> # cat /proc/mdstat
> Personalities : [raid1]
> md10 : active raid1 sde[1] sdd[0]
>   499976512 blocks super 1.2 [2/2] [UU]
>   bitmap: 0/4 pages [0KB], 65536KB chunk

RAID 1 has no "layout" (for RAID, that usually refers to striping in
RAID levels 0/5/6), so there's nothing for a filesystem to detect or
optimize for.

The chunk size above is for the md-RAID write-intent bitmap; that's not
exposed information (for any RAID system that I'm aware of, software or
hardware) or something that filesystems can optimize for.

-- 
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Re: [CentOS] xfs not getting it right?

[Stephen John Smoogen]

On 20 September 2017 at 10:47, hw  wrote:
>
> Hi,
>
> xfs is supposed to detect the layout of a md-RAID devices when creating the
> file system, but it doesn´t seem to do that:
>
>
> # cat /proc/mdstat
> Personalities : [raid1]
> md10 : active raid1 sde[1] sdd[0]
>   499976512 blocks super 1.2 [2/2] [UU]
>   bitmap: 0/4 pages [0KB], 65536KB chunk
>
>
> # mkfs.xfs /dev/md10p2
> meta-data=/dev/md10p2isize=512agcount=4, agsize=30199892
> blks
>  =   sectsz=512   attr=2, projid32bit=1
>  =   crc=1finobt=0, sparse=0
> data =   bsize=4096   blocks=120799568, imaxpct=25
>  =   sunit=0  swidth=0 blks
> naming   =version 2  bsize=4096   ascii-ci=0 ftype=1
> log  =internal log   bsize=4096   blocks=58984, version=2
>  =   sectsz=512   sunit=0 blks, lazy-count=1
> realtime =none   extsz=4096   blocks=0, rtextents=0
>
>
> # mkfs.xfs -f -d su=64m,sw=2 /dev/md10p2
> meta-data=/dev/md10p2isize=512agcount=16, agsize=7553024
> blks
>  =   sectsz=512   attr=2, projid32bit=1
>  =   crc=1finobt=0, sparse=0
> data =   bsize=4096   blocks=120799568, imaxpct=25
>  =   sunit=16384  swidth=32768 blks
> naming   =version 2  bsize=4096   ascii-ci=0 ftype=1
> log  =internal log   bsize=4096   blocks=58984, version=2
>  =   sectsz=512   sunit=8 blks, lazy-count=1
> realtime =none   extsz=4096   blocks=0, rtextents=0
>
>
> The 64MB chunk size was picked my mdadm automatically.  The device is made
> from two disks, and xfs either doesn´t figure that out, or it decided to
> ignore the layout of the underlying RAID.
>
> Am I doing something wrong here, or is xfs in Centos somehow different?
> Do, or must, we always specify the apporpriate values for su and sw or
> did xfs ignore them because what it picked is better?
>

I don't know enough to answer, but I do have a question.. what were
you expecting xfs to do (and what filesystems do that?) Thanks

-- 
Stephen J Smoogen.
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[CentOS] xfs not getting it right?

[hw]

Hi,

xfs is supposed to detect the layout of a md-RAID devices when creating the
file system, but it doesn´t seem to do that:


# cat /proc/mdstat
Personalities : [raid1]
md10 : active raid1 sde[1] sdd[0]
  499976512 blocks super 1.2 [2/2] [UU]
  bitmap: 0/4 pages [0KB], 65536KB chunk


# mkfs.xfs /dev/md10p2
meta-data=/dev/md10p2isize=512agcount=4, agsize=30199892 blks
 =   sectsz=512   attr=2, projid32bit=1
 =   crc=1finobt=0, sparse=0
data =   bsize=4096   blocks=120799568, imaxpct=25
 =   sunit=0  swidth=0 blks
naming   =version 2  bsize=4096   ascii-ci=0 ftype=1
log  =internal log   bsize=4096   blocks=58984, version=2
 =   sectsz=512   sunit=0 blks, lazy-count=1
realtime =none   extsz=4096   blocks=0, rtextents=0


# mkfs.xfs -f -d su=64m,sw=2 /dev/md10p2
meta-data=/dev/md10p2isize=512agcount=16, agsize=7553024 blks
 =   sectsz=512   attr=2, projid32bit=1
 =   crc=1finobt=0, sparse=0
data =   bsize=4096   blocks=120799568, imaxpct=25
 =   sunit=16384  swidth=32768 blks
naming   =version 2  bsize=4096   ascii-ci=0 ftype=1
log  =internal log   bsize=4096   blocks=58984, version=2
 =   sectsz=512   sunit=8 blks, lazy-count=1
realtime =none   extsz=4096   blocks=0, rtextents=0


The 64MB chunk size was picked my mdadm automatically.  The device is made
from two disks, and xfs either doesn´t figure that out, or it decided to
ignore the layout of the underlying RAID.

Am I doing something wrong here, or is xfs in Centos somehow different?
Do, or must, we always specify the apporpriate values for su and sw or
did xfs ignore them because what it picked is better?

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