Bristol Digest, Vol 549, Issue 1

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Today's Topics:

1. Mirror ssd to hdd (Andrew)
2. Re: Mirror ssd to hdd (Alex Butcher)
3. Re: Mirror ssd to hdd (Steve Netting)
4. Re: Mirror ssd to hdd (Andrew)
5. Re: Mirror ssd to hdd (Shane McEwan)
6. Re: Mirror ssd to hdd (John Daragon)


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Message: 1
Date: Wed, 07 May 2014 09:37:47 +0100
From: Andrew <andrewsoltau@gmail.com>
To: bristol@mailman.lug.org.uk
Subject: [bristol] Mirror ssd to hdd
Message-ID: <5369F0DB.3050004@1dtv.com>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"

Greetings luggers

Building a new box with particular attention to backups after a recent
ssd sudden fail. (NOW I know all about sandforce. Ugh!)

I'm running Debian Wheezy. What I want is to run ssd for speed, but
mirror it to hdd. What I am after is an instantly bootable backup drive
so that if and when this repeats I can carry on working where I left
off. (I am also buying Samsung ssd to minimise likelihood.) Obviously
raid 1 defeats the objective. And I am assuming that if I simply rsync
the backup drive with the ssd it may / will become unbootable. Can I put
grub on its own tiny partition and solve that problem?

The only obvious solution I can think of is to boot from a pen drive or
third sata once a week and dd the / partition, but I would love to find
a less interactive solution. I have home on a separate partition, so I
can rsync that no problem - already do that which is why the failure was
not too catastrophic. If just the grub stuff does not work - I assume
grub has to access a specific address on the drive (or not?) - I
wondered if putting key directories in the small grub partition might work.

Or is there some Debian magic I can use for all this?

Andrew
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Message: 2
Date: Wed, 7 May 2014 10:04:43 +0100 (BST)
From: Alex Butcher <lug@assursys.co.uk>
To: andrew@1dtv.com, Bristol and Bath Linux User Group
<bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Mirror ssd to hdd
Message-ID: <alpine.LFD.2.03.1405070958590.7603@nffheflf.pb.hx>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed

On Wed, 7 May 2014, Andrew wrote:

> I'm running Debian Wheezy. What I want is to run ssd for speed, but mirror
> it to hdd. What I am after is an instantly bootable backup drive so that if
> and when this repeats I can carry on working where I left off. (I am also
> buying Samsung ssd to minimise likelihood.) Obviously raid 1 defeats the
> objective.

Why do you say that? Obviously, I/O to/from the HDD will suffer a performance
penalty relative to the SSD, but I think in practice the SSD will pick up
most of the I/O.

However, if that's a concern, you probably want to look into something like
<http://bcache.evilpiepirate.org/> or
<http://en.wikipedia.org/wiki/Dm-cache>.

> Andrew

HTH,
Alex



------------------------------

Message: 3
Date: Wed, 7 May 2014 10:24:26 +0100
From: "Steve Netting" <steve@track3.org.uk>
To: andrew@1dtv.com, "Bristol and Bath Linux User Group"
<bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Mirror ssd to hdd
Message-ID:
<125304506d2a12437c482395ea3b00f8.squirrel@webmail.track3.org.uk>
Content-Type: text/plain;charset=iso-8859-1

> Greetings luggers
>
> Building a new box with particular attention to backups after a recent
> ssd sudden fail. (NOW I know all about sandforce. Ugh!)
>
> I'm running Debian Wheezy. What I want is to run ssd for speed, but
> mirror it to hdd. What I am after is an instantly bootable backup drive
> so that if and when this repeats I can carry on working where I left
> off. (I am also buying Samsung ssd to minimise likelihood.) Obviously
> raid 1 defeats the objective.

Why not RAID 1? You should see read speeds reflective of SSD performance.
Writes may suffer but with sufficient RAM you're probably buffering
writes anyway.

mdadm --create -n 2 -l 1 /dev/md0 /dev/<SSD> -W /dev/<HDD>

-W, --write-mostly
subsequent devices listed in a --build, --create, or --add command
will be flagged as ?write-mostly?. This is valid for RAID1 only and
means that the ?md? driver will avoid reading from these devices if at
all possible. This can be useful if mirroring over a slow link.

I'd be interested to hear how this works in practice.

The other option would be something like lsyncd (live rsync) but
personally I'd keep it simple with mdadm :)


Steve





------------------------------

Message: 4
Date: Wed, 07 May 2014 10:43:57 +0100
From: Andrew <andrewsoltau@gmail.com>
To: Bristol and Bath Linux User Group <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Mirror ssd to hdd
Message-ID: <536A005D.3080507@1dtv.com>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"

On 07/05/14 10:24, Steve Netting wrote:
>> Greetings luggers
>>
>> Building a new box with particular attention to backups after a recent
>> ssd sudden fail. (NOW I know all about sandforce. Ugh!)
>>
>> I'm running Debian Wheezy. What I want is to run ssd for speed, but
>> mirror it to hdd. What I am after is an instantly bootable backup drive
>> so that if and when this repeats I can carry on working where I left
>> off. (I am also buying Samsung ssd to minimise likelihood.) Obviously
>> raid 1 defeats the objective.
> Why not RAID 1? You should see read speeds reflective of SSD performance.
> Writes may suffer but with sufficient RAM you're probably buffering
> writes anyway.
>
> mdadm --create -n 2 -l 1 /dev/md0 /dev/<SSD> -W /dev/<HDD>
>
> -W, --write-mostly
> subsequent devices listed in a --build, --create, or --add command
> will be flagged as 'write-mostly'. This is valid for RAID1 only and
> means that the 'md' driver will avoid reading from these devices if at
> all possible. This can be useful if mirroring over a slow link.
>
> I'd be interested to hear how this works in practice.
>
> The other option would be something like lsyncd (live rsync) but
> personally I'd keep it simple with mdadm :)
>
>
> Steve
Thank you. This looks very cool. I have to say I don't particularly want
to guinea pig interesting new developments! But from what you say this
looks highly workable.

Andrew
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Message: 5
Date: Wed, 07 May 2014 10:46:08 +0100
From: Shane McEwan <shane@mcewan.id.au>
To: bristol@mailman.lug.org.uk
Subject: Re: [bristol] Mirror ssd to hdd
Message-ID: <536A00E0.8040607@mcewan.id.au>
Content-Type: text/plain; charset=UTF-8

On 07/05/14 09:37, Andrew wrote:
> I'm running Debian Wheezy. What I want is to run ssd for speed, but
> mirror it to hdd. What I am after is an instantly bootable backup drive
> so that if and when this repeats I can carry on working where I left
> off. (I am also buying Samsung ssd to minimise likelihood.) Obviously
> raid 1 defeats the objective. And I am assuming that if I simply rsync
> the backup drive with the ssd it may / will become unbootable. Can I put
> grub on its own tiny partition and solve that problem?

You could try an LVM mirror[1]. It will only mirror the volume so you'd
still need to copy your Grub boot sector[2]. Note that an LVM mirror is
different to LVM RAID 1. I'm not sure how the mirror will impact your
performance. I would recommend doing some benchmarks. I suspect you'll
have a performance penalty with write operations because the operation
won't complete until the copy has been written to the HDD.

My suggestion would be to get a second SSD and run RAID 1. :-)

[1]
https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/mirror_create.html
[2] http://www.cyberciti.biz/faq/howto-copy-mbr/

Shane.



------------------------------

Message: 6
Date: Wed, 7 May 2014 11:20:20 +0000
From: John Daragon <john@blacklabs.co.uk>
To: "andrew@1dtv.com" <andrew@1dtv.com>, Bristol and Bath Linux User
Group <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Mirror ssd to hdd
Message-ID: <0B4DF93CCC620247AC7795BCE93F9D635993B9@MBX01.npxe.lan>
Content-Type: text/plain; charset="us-ascii"

There's a really good overview of hybrid SSD/HDD RAID1 (a configuration I'm particularly keen on in other OS implementations...) at http://tansi.info/hybrid/

jd

From: bristol-bounces@mailman.lug.org.uk [mailto:bristol-bounces@mailman.lug.org.uk] On Behalf Of Andrew
Sent: 07 May 2014 09:38
To: bristol@mailman.lug.org.uk
Subject: [bristol] Mirror ssd to hdd

Greetings luggers

Building a new box with particular attention to backups after a recent ssd sudden fail. (NOW I know all about sandforce. Ugh!)

I'm running Debian Wheezy. What I want is to run ssd for speed, but mirror it to hdd. What I am after is an instantly bootable backup drive so that if and when this repeats I can carry on working where I left off. (I am also buying Samsung ssd to minimise likelihood.) Obviously raid 1 defeats the objective. And I am assuming that if I simply rsync the backup drive with the ssd it may / will become unbootable. Can I put grub on its own tiny partition and solve that problem?

The only obvious solution I can think of is to boot from a pen drive or third sata once a week and dd the / partition, but I would love to find a less interactive solution. I have home on a separate partition, so I can rsync that no problem - already do that which is why the failure was not too catastrophic. If just the grub stuff does not work - I assume grub has to access a specific address on the drive (or not?) - I wondered if putting key directories in the small grub partition might work.

Or is there some Debian magic I can use for all this?

Andrew
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End of Bristol Digest, Vol 549, Issue 1
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