Re: Raid support in debian.
On Tuesday 16 January 2001 22:28, Ola Lundqvist wrote:
> I'm working on a project (at work) to make debian support raid in a
> smooth and good way. We have made some hacked boot disks so that
> you can fix things before you start the actual install. But this is
> probably not the best solution.
I've just posted a message offering to do the same. But I'm quite busy and
would be more than happy to let you do it. I will of course contribute to
your work wherever I can!
> * Before partitioning ask if you want to enable raid. (possibly only ask
> if there are just one hard disk)
>
> * Then let the user paritionate the first raid disk (tell the user to
> use the special raid partition type in the dialog above).
>
> * After that ask on which disk(s) this setup should be duplicate.
>
> * Copy over the tables to these disks.
This doesn't solve the following issues:
RAID on two disks of different sizes.
RAID on disks with different IDE geometry.
RAID on IDE and SCSI disks (not even sure if the kernel supports this yet).
You purchase a machine with one hard drive and want to buy a second hard
drive next week/month/year to take advantage of RAID.
> * Create a raid table.
Creating a RAID table with both drives online means syncing them. This can
take several hours if nothing else is being done to the system. This gives
the user two options:
1) Leave the system for a few hours until RAID is done before continuing the
installation.
2) Install while RAID is syncing which makes the install take significantly
longer, and then have a post-install reboot before the RAID sync is finished.
NB You could reduce the maximum sync speed to some small number to alleviate
this.
I suggest that the solution is to create a single-disk degraded mirror.
As for the suggestion of RAID5 support, RAID5 has the following problems:
It requires at least three disks (RAID5 with 2 disks is RAID1) and three
cables for proper operation. Most people don't have such hardware. Everyone
has a machine with two IDE connectors and buying two IDE drives is quite
affordable.
RAID5 sucks for write speed of small blocks of data (metadata operations).
RAID1 devices can be mounted directly without RAID, this is a significant
advantage for reliability as it gives you yet another way of recovering hosed
systems. RAID5 doesn't have this.
RAID-5 really isn't suitable for most people who want/need RAID and is not
suitable for a root file system. RAID-5 has big problems for /boot (which
could be solved) and for recovery situations should not be used for the root
FS either.
I recommend giving no support for RAID-5 in the RAID installation for these
reasons and because it means significantly more work in writing the
installation programs (which means more testing requirements and more
possibilities for things to fail).
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