Bristol Digest, Vol 549, Issue 2

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Today's Topics:

1. Re: Mirror ssd to hdd (Steve King)
2. Re: Mirror ssd to hdd (nick robinson)
3. Re: Mirror ssd to hdd (Andrew)


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Message: 1
Date: Wed, 7 May 2014 14:25:20 +0100
From: "Steve King" <debian@invux.com>
To: "Bristol and Bath Linux User Group" <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Mirror ssd to hdd
Message-ID:
<efbd64a2ba21deebe52b27809d23926d.squirrel@dazzle.invux.com>
Content-Type: text/plain;charset=iso-8859-1


> 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.


I have used exactly this configuration.

And, with the exception of the GRUB and EFI configurations, which I have
yet to work out how to install on /dev/sdb, in such a way that the system
will boot after a catastrophic failure of /dev/sda, my data remained safe
when the SSD failed.

Yes, you get very fast reads, I have noticed it particularly with:
1. Restoring two 4 Gigabyte xen images when rebooting Dom0
2. Searching my IMAP mail server

munin claims microsecond seek statistics.

However writes are HDD speed, so it is will not be much use for a
write-intensive filesystem.

I have yet to decide whether the speed improvements the SSD are a just
nice to have, or actually offer material benefit over what I would enjoy
from installing the disk somewhere else.

YMMV

--
Steve




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Message: 2
Date: Wed, 7 May 2014 14:27:24 +0100
From: nick robinson <nick@njrobinson.net>
To: andrew@1dtv.com, Bristol and Bath Linux User Group
<bristol@mailman.lug.org.uk>
Subject: Re: [bristol] Mirror ssd to hdd
Message-ID:
<CADo8qK5HEGgDjpFLpUfsiqteHrws+ZXS7pwuY7PGSSGKKqkH7A@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"

This may be a solution specific to my version of bios but what you could do
is ..

make an initial mirror of the disk (use dd or something similar)
set up rsync for periodic disk syncs
set the boot order in bios to start with your ssd, if it fails it should
automatically boot off the next available disk


On 7 May 2014 09:37, Andrew <andrewsoltau@gmail.com> 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. 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
>
> _______________________________________________
> Bristol mailing list
> Bristol@mailman.lug.org.uk
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Message: 3
Date: Thu, 08 May 2014 09:20:40 +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: <536B3E58.3080601@1dtv.com>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"

On 07/05/14 10:04, Alex Butcher wrote:
> 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
My problem is solved, but I'm still curious about b cache and dm cache.
Do these actually hold a copy of the hdd on the ssd, or just act as
cache like the ssd component of a combo drive?

Andrew
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