RAID Info
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Subject: Re: multiple transaction logs/RAID
From: Mark Lillywhite
Date: 1998-06-08 19:08:32
> My experience with RAID was that at a previous company I had my home
> directory on an NFS-mounted RAID system. It could be quite slow at times.
> Also, we lost a day's work and had to restore from tape because the RAID
> system had some kind of electrical problem that corrupted more than one
> disk.
I had quite a lot of experience with RAID database systems a few years ago,
though unfortunately not with Solid. In general, RAID-5 arrays will increase
read speed quite dramatically compared to a similarly loaded single disk drive
(more spindles) but write speed takes something like two-three times as long
(because RAID5 does >1 read and two writes). RAID is more reliable than single
disk subsystems, but it's still not 100% uptime, as you've discovered. However
I was very impressed with the RAID system I was using (mostly DG CLARiiON,
though some HP as well, though HP's servers left a lot to be desired in 1995).
You would never, for example, store a write-intensive database on RAID, and you
should never store things like transaction logs because they are basically
write-only. In fact, as you say, writing the logs to a separate disk on a
separate controller is the best way to store the logs reliably. I don't know
what Solid does if the log filesystem dies, though; maybe you could store the
logs on a mirrored filesystem and the database on a separate RAID-5 filesystem.
Mirrors don't normally cause much of a performance hit, in our experience, and
you could even use software mirroring between two controllers, if you were
really paranoid.
> Perhaps that was unusual, but as a result, I'm not willing to the
> reliability of *any* hardware for granted (which is why I would really
> prefer to have more than one transaction log and use separate SCSI
> controllers going to separate disks).
>
> But perhaps I will take a chance on a RAID. So, I would like to know how
> using a RAID would affect Solid's performance, and since RAID systems tend
> to cache writes, I'd like be sure that when a transaction is committed,
> the transaction log is written all the way to disk.
The RAID systems I've used did write caching to NVRAM and was still transparent
to the application (i.e. if the power failed, any sync'd writes were still
available when power was restored). I think that Solid (and most other
databases) will expect that if the OS says the data's written, then it is; I
don't know how you could do transaction recovery from a database where the data
was supposedly committed, but isn't; if a checkpoint doesn't really checkpoint,
I suspect you'll lose data no matter how many logs you have.
These are just random thoughts about the problem, in no particular order :)
Mylex RAID controllers; building hot-swap drives
> Theoretically, any SCSI drive can be installed in a hot-swap chassis.
> However, to prevent damage to the drive, you need a SCSI adapter that allows
> you to "down" the drive prior to removal, and bring it "up" after you insert
> it. You also need the appropriate software to use this feature of the card.
> Note that hot-swap is normally a feature found only on RAID hardware, and to
> the best of my knowledge, there are very few RAID adapters that are natively
> supported in Linux, and none of those provide feature software for Linux.
Although its not supported out of the box by RedHat 5.2, I recently
setup a system using a fairly inexpensive RAID controller - a Mylex
AcceleRAID 250, using the drivers from www.dandelion.com. Works great!
And native utilities are on the way, they tell me. Using the DAC960
driver with this card, I simulated drive failures on my RAID array by
powering down various drives, pulling them out, and then putting them
back into the chassis. Works well.
If you want to pull a live drive under Linux though, all you need to do
is do an unmount the drive first. No specific "down" command is
required. You DO need a hotswap drive carrier for the drive though, in
order to support this without trashing the SCSI bus.
Eric Bohn wrote:
>
> >Would I need any specific hardware to do this? Does the card itself have
> to
> >support hot swapping or will any card work? Does the harddrive have to
> >support hot swapping? I've seen hot swap UW SCSI drive bays for sale but
> >will any UW SCSI harddrive work in them? Will I need to do this through
> >Win98 instead of Linux?
I have recently used some hot-swap drive carriers and receiving frames
(for 1/2 hieght 5.25" drive bays) from Kingston to do this. They would
take any SCSI UW drive - the restriction was though that you have to do
single ended to support hot swapping. The drive carriers I used were
Kingston DataExpress units. To support HOT swapping, you have to order
the "hot-swap module" for the carrier though. Carriers cost about $200,
and the hot-swap module (a little circuit card) costs about $50. The
units have a keylock and LED display on the front. When you turn the key
to unlock the drive, it waits until the SCSI bus is silent before
disconnecting the drive from the bus. It then waits until the drive
spins down (typically 30 seconds) before telling you its okay to pull
the carrier. All in all, a very rugged, well built unit.
Megatrends MegaRAID controllers
> FYI the drivers are at ftp://ftp.megatrends.com/MegaRaid/Drivers/linux
>
> American Megatrends Adds Linux System
> By BusinessWire
> 23-Oct-1998
>
> ATLANTA--(BUSINESS WIRE)--Oct. 23, 1998-- American Megatrends, Inc., a
> leading supplier of core technology solutions, today announced support for
> the Linux Operating System.
> Software drivers for American Megatrends MegaRAID(R) controllers are now
> available for the latest release of the Red Hat Linux operating system,
> version 5.2. AMI now offers Linux-ready drivers for its low-end MegaRAID
> cards such as MegaRAID Express and MegaRAID Express Plus, as well as its
> high-end cards such as the MegaRAID Ultra, MegaRAID Ultra GT, and MegaRAID
> Ultra2 LVD.
> AMI is also providing a RAID management utility called MegaRAID Manager,
> which will work under Linux to configure and manage MegaRAID based
> systems. This utility will be released the first week of November.
> Currently, MegaRAID users are able to use AMI's award-winning OS
> independent "BIOS Configuration Utility" for RAID configuration and
> management.
> The Linux operating system is developed under General Public License
> (GPL). Linux runs on three leading computing platforms: Intel compatible
> PCs, Digital Alpha Computers, and Sun SPARC equipment.
> In addition to Linux, AMI currently supports a number of network operating
> systems, including Windows NT(R), Novell Netware(R), OS/2(R), SCO UNIX(R),
> UnixWare(R), Solaris(tm), Banyan VINES(tm), and OpenBSD.
> Founded in 1985, American Megatrends Inc. (AMI) provides OEMs, resellers
> and systems integrators with leading-edge computer technologies. This
> privately held company, best known for its Basic Input/Output Systems or
> BIOS, also supplies its hardware, software, utilities and RAID controllers
> to over 55% of today's high-tech manufacturers.
> Headquartered in Atlanta, Georgia, AMI has locations in the US and
> throughout the world including Munich, London, Taipei, Madras and Tokyo.
> OCT 23,1998 10:49 EASTERN
> Copyright 1998 by BusinessWire. All rights reserved. News provided by
> COMTEX.
Everstore external RAID systems
I'm using an external RAID5 box from micropal (www.micropalcorp.com) right
now in a squid proxy server / ethernet router for our wireless ethernet
network and customers.
A hard drive died, and LEDs changed colors and the thing started beeping.
I pulled out the bad hard drive as indicated by the row of LEDs, put a new
one in the bracket, and plugged it back in. The box rebuilt it and was
happy again. Since this is handled by the external raid and is completely
independent of the server, nothing went down, no delays or downtime
resulted, and no commands, utilities, or reboots had to be issued on the
server (RedHat 5.1).
External RAID is cool stuff.
I'll probably buy another external RAID5 box for the next super-important
computer I build.
> Anybody running a version of RAID under RH5/5.1 right now?
>
> I'm looking for war stories with software vs hardware raid.
>
> Drew
>
> it appears that around 9/30/98 9:58 PM, Lance A. Brown said:
>
> >I have tested the RAID-5 support provided by the DPT brand cards
> >running under Linux 5.0 on Intel hardware. Worked great. Only thing
> >I didn't like was having to boot MSDOS in order to run the
> >configuration utility. DPT didn't have a native linux configuration
> >util at the time.
Hardware vs software RAID: comparison
On Thu, 1 Oct 1998, Drew Mouton wrote:
> Anybody running a version of RAID under RH5/5.1 right now?
>
> I'm looking for war stories with software vs hardware raid.
Yep.
I've used the ICP-Vortex GDT line extensively, and benchmarked them
against the software RAID available in the then-current 2.1.119 kernel.
The machine: dual-processor Pentium II-400 (the fastest available at that
time) with 512mb of RAM and fifteen 9.1GB Cheetah hard drives variously
attached to a 5-channel GDT controller or to the on-board NCR SCSI
controller (Intel Nightshade motherboard). The # of hard drives made it
easy to make direct (VERY direct) comparisons on various RAID setups.
Results:
On RAID0 and RAID1, the difference in performance between software RAID
and hardware RAID was insignificant. Both managed around 23 megabytes per
second sustained throughput on a 3-drive array. I'm at a loss as to what
the limiting factor is on throughput here, since a single 9.1GB Cheetah
can sustain 18 megabytes per second.
On RAID5, the advantage went decidedly to the hardware RAID controller.
I'd figured that a 400mhz P-II could certainly out-compute the 66mhz i960
on the RAID controller when it came to decoding the ECC and figuring the
stripes. Nope. On RAID5, sustained throughput of software RAID went down
to 19MBytes/second while the hardware RAID remained at around 23 megabytes
per second.
On robustness there is no comparison. On the GDT, you can pull a drive
tray out and the machine will beep at you but keep on going. Put the drive
tray back in and the GDT rebuilds that drive. You don't have to run a
program there, unless it's a totally different drive in which case you
have to run 'gdtmon' to add the drive to the array (but you dont' have to
be in single user mode or run any utilities to actually build the array,
the array rebuilds itself in the background, totally transparent to you).
On software RAID, you have to basically bring the system down to single
user mode to rebuild a drive, plus you can't boot or have your root on a
software RAID drive (at the moment -- yes, I understand that is changing).
ICP Vortex controllers
I'm just installing yet a RH5.1 on an ICP-Vortex GDT6117RP.
RAID-1, but with a RAIDYNE also RAID-5 is supported.
Monitoring is possible with the 'gdtmon' util, runs under
Linux (download it on their website).
http://www.icp-vortext.com
Runs perfect...