Linux and RAID striping

Linux and RAID striping

Post by Sam Trenhol » Sat, 16 May 1998 04:00:00



The current situtation with Linux is when my mission critical Linux server
has a bad hard disk, I have to go to the server's site, replace hard disk
then restore from backup and/or work around the partitons in question not
being there, and make excuses to my users about why the system went down.

What I would like to ideally see is the md drivers being able to do the
following:

* 100% mirror redunancy.  When a hard disk gues the way of the dodo, nothing
  changes except root gets mail saying "Hard disk failure".

* I add a note to /etc/motd that there will be 20 minutes of down time as we
  swap hard disks.

* I swap hard disks.

* The data from the working hard disk is automagiclly mirrored to the new
  hard disk.

Total downtime is 20 minutes, at a preplanned time, instrad of however long
it takes between the time the hard disk dies and I am able to get to the
server room to replace the hard disk in question.

Is Linux able to do this yet?

- Sam

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Linux and RAID striping

Post by Robert Hyat » Sat, 16 May 1998 04:00:00


: The current situtation with Linux is when my mission critical Linux server
: has a bad hard disk, I have to go to the server's site, replace hard disk
: then restore from backup and/or work around the partitons in question not
: being there, and make excuses to my users about why the system went down.

: What I would like to ideally see is the md drivers being able to do the
: following:

: * 100% mirror redunancy.  When a hard disk gues the way of the dodo, nothing
:   changes except root gets mail saying "Hard disk failure".

: * I add a note to /etc/motd that there will be 20 minutes of down time as we
:   swap hard disks.

: * I swap hard disks.

: * The data from the working hard disk is automagiclly mirrored to the new
:   hard disk.

: Total downtime is 20 minutes, at a preplanned time, instrad of however long
: it takes between the time the hard disk dies and I am able to get to the
: server room to replace the hard disk in question.

: Is Linux able to do this yet?

: - Sam

Buy yourself a raid controller...  And set it up in the way you
mentioned...  I've not tried the "hot swap" stuff myself (my skin
crawls at the thought, being an old hardware person) but I know
of places that use it reliably...  not in Linux, but in general.

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Linux and RAID striping

Post by David Bullo » Sat, 16 May 1998 04:00:00




Quote:>Buy yourself a raid controller...  And set it up in the way you
>mentioned...  I've not tried the "hot swap" stuff myself (my skin
>crawls at the thought, being an old hardware person) but I know
>of places that use it reliably...  not in Linux, but in general.

We preinstall and support Linux on our servers (on request) where I
work.  We use hotswap cages with the DPT SmartRAID IV cards.
(www.land-5.com)

The only drawback I see with the DPT is that DPT has shown no interest
in implementing a program to start the rebuild without dropping to DOS
(or another OS).  Their tech support guy said it's because they would
have to distribute source, but he was so obviously anti-linux I didn't
see a point in trying to enlighten him, and their tech support manager
has not yet returned my calls.

Dave

 
 
 

Linux and RAID striping

Post by Sam Trenhol » Sat, 16 May 1998 04:00:00


Quote:>Buy yourself a raid controller...  And set it up in the way you
>mentioned...  

Thanks for the suggestion.  Since posting that article, I got Netscape on
my new Linux system going, and found this gem:

        http://linas.org/linux/raid.html

The answer is "yes, Linux can do this"  The code is not part of the
kernel proper, but is considered "mostly stable".

- Sam

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Linux and RAID striping

Post by Chris Bradfiel » Mon, 18 May 1998 04:00:00


:       http://linas.org/linux/raid.html

: The answer is "yes, Linux can do this"  The code is not part of the
: kernel proper, but is considered "mostly stable".

I've ran raid-0 just to mess around with the md drivers in the newer
development kernels and have found it to be much slower (few seconds) on
large, single user copies.  I would expect raid-5 to be REALLY low considering
it is software.  If you are using these machines in a production environment,
I would definately recommend you purchase a RAID controller rather than
messing with software RAID.  It just runs too slow.

- Chris

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Linux and RAID striping

Post by H. Peter Anv » Tue, 19 May 1998 04:00:00




In newsgroup: comp.os.linux.development.system

Quote:

> What I would like to ideally see is the md drivers being able to do the
> following:

> * 100% mirror redunancy.  When a hard disk gues the way of the dodo, nothing
>   changes except root gets mail saying "Hard disk failure".

> * I add a note to /etc/motd that there will be 20 minutes of down time as we
>   swap hard disks.

> * I swap hard disks.

> * The data from the working hard disk is automagiclly mirrored to the new
>   hard disk.

> Total downtime is 20 minutes, at a preplanned time, instrad of however long
> it takes between the time the hard disk dies and I am able to get to the
> server room to replace the hard disk in question.

> Is Linux able to do this yet?

I think the md device in RAID-1 mode should be able to do exactly this.

        -hpa
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Linux and RAID striping

Post by Gordon Sco » Tue, 19 May 1998 04:00:00


: We preinstall and support Linux on our servers (on request) where I
: work.  We use hotswap cages with the DPT SmartRAID IV cards.
: (www.land-5.com)

I'll give a vote to DPT controllers, too. I've used their raids on
Windows and the non-raids under both Linux and SCO unix and they're
good product (IMHO). I haven't tried their SCO admin tools (with iBCS2),
or DOS tools under DOSEMU. Probably I should -- just to see!

--
Gordon Scott             Opinions expressed are my own.


Linux  ...............   Because I like to _get_ there today.

 
 
 

Linux and RAID striping

Post by James Youngma » Tue, 19 May 1998 04:00:00


  >> * The data from the working hard disk is automagiclly mirrored to the new
  >> hard disk.
  >>
  >> Total downtime is 20 minutes, at a preplanned time, instrad of however long
  >> it takes between the time the hard disk dies and I am able to get to the
  >> server room to replace the hard disk in question.
  >>
  >> Is Linux able to do this yet?
  >>

  hpa> I think the md device in RAID-1 mode should be able to do exactly this.

I thought that the RAID1 support in md didn't allow  you to bring back
up the new disk while the system was running?  Certainly, my system is
set up to run 'ckraid --fix' (if it's required) at bootup time, before
the 'fsck' of  the local filesystems, which is  a pain (since it takes
40 minutes for a 4Gb disk).

 
 
 

Linux and RAID striping

Post by bill davids » Tue, 19 May 1998 04:00:00




| I've ran raid-0 just to mess around with the md drivers in the newer
| development kernels and have found it to be much slower (few seconds) on
| large, single user copies.  I would expect raid-5 to be REALLY low considering
| it is software.  If you are using these machines in a production environment,
| I would definately recommend you purchase a RAID controller rather than
| messing with software RAID.  It just runs too slow.

Since I run it (happily) on a number of production machines, I suspect
that you have the config mismatched to the data access pattern. This is
*not* plug and play configuration, you have to understand the access
pattern. I don't know of any hardware controller which allows you to use
one stripe size on one set of partitions over several drives, and
another stripe size for another data set striped over the same drives.

And if you "messed around" using IDE, I hope you used one drive per
controller and not two drives on the same cable.

Used correctly I can get an increase of 60-80% in average transfer rate
for big files, or the same increase percentage in tps on small database
lookups, all using software.
--

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  case, people would die."
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Linux and RAID striping

Post by Rik van Rie » Tue, 19 May 1998 04:00:00




> : The answer is "yes, Linux can do this"  The code is not part of the
> : kernel proper, but is considered "mostly stable".

> I've ran raid-0 just to mess around with the md drivers in the newer
> development kernels and have found it to be much slower (few seconds) on
> large, single user copies.

This depends on your usage pattern. Also, the current RAID
implementation won't do readaheads that cross device
boundaries. This might be the limiting factor...

Quote:>                 I would expect raid-5 to be REALLY low considering
> it is software.

Not necessarily. When reading from a RAID-5 volume, you only
need to read data from the disk where it is located. There's
no need to recalculate anything, except when one of the disks
is down.

Only writing will be a little bit slower, but that shouldn't
be much of an issue, since most people don't just write random
data. They tend to generate the data first; compared to the
generating, the RAID parity calculation is lost in the noise.

Only for really-very-very-high performance or data streaming
you'd want an expensive hardware RAID solution. For a simple
file server (mostly read-only) or scientific station (costly
data generation so RAID stuff is negligable) software RAID
will do.

Only for database/transaction servers and video streaming
you'll definately want hardware RAID.

Rik.
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