RAID and journaling fs - a bad idea?

**ThomasX** · 03-18-2005, 03:11 PM

I just wanted to set up a RAID1, to have something I can count on. But then I started to think about it: What happens, if you have a power failure (hard switch off) in a system with e.g. reiserfs and RAID1? On the one HD, the writing transaction may got finished until poweroff, and on the 2nd HD the write transaction was still ongoing. So at the next poweron, the write gets rolled back on the 2nd HD, but not on the 1st HD. Now you have a mirrored system with not identical data.

Do you have any idea, how (or if) this is solved?

**Dave_Bechtel** · 03-20-2005, 06:24 PM

--I'm sure the designers thought of this, as Reiserfs is used in major production systems.

--However, you can always go here ( http://www.namesys.com/ ) and check the FAQ, or send them an email.

--My take on it, is that you're better off WITH journalling, than without. But I also recommend buying a UPS.

--BTW, can anyone recommend a good UPS with Linux support?

Originally Posted by ThomasX

I just wanted to set up a RAID1, to have something I can count on. But then I started to think about it: What happens, if you have a power failure (hard switch off) in a system with e.g. reiserfs and RAID1? On the one HD, the writing transaction may got finished until poweroff, and on the 2nd HD the write transaction was still ongoing. So at the next poweron, the write gets rolled back on the 2nd HD, but not on the 1st HD. Now you have a mirrored system with not identical data.

Do you have any idea, how (or if) this is solved?

**Cuddles** · 03-20-2005, 07:58 PM

Dave Bechtel,

Not quite sure on this, but, as for your querry on UPS's...

I dont think you can go wrong with APC -=- I have been using them for many years, without problems.

I dont have the seriel interface running, I just use the power and phone plugs, but, the units are "rock solid", and have held me through every-other-minute of brown-outs, used to live in Tucson, Arizona, when the thunder and lightening storms get bad during monsoon season.

Considering how big APC has gotten, I can not imagine that they havent, at least, ported, there software over to Linux, if not, created completely new support software for it... Possibly check the web site, and hit up there tech support ?

As for just the hardware, I am a devoted buyer of APC, and feel quite safe having used for more than twenty years, and more than 3 systems - not to mention there warrentee that backs there products.

Ms. Cuddles

**koaschten** · 03-20-2005, 08:29 PM

apc does support linux, at least on the serial port, there even is the software to monitor it, not sure if it has a client, but the demon works like a charm, monitoring it from a windows workstation.

**Dave_Bechtel** · 03-21-2005, 09:30 AM

--Thanks for the UPS advice!

**ThomasX** · 03-21-2005, 01:41 PM

back to my orig. question: I got an answer:

I assumed that RAID is on top of the fs, but I got told that actually the fs is on top of the RAID module. Therefore there is only *one* fs "instance" involved, which either gets rolled back or not.

However, a new question arises now: If a journaling fs gives savety on power failure, but a raid software is in between, does it probably break your power off savety for the sake of HD HW failure savety?

**ThomasX** · 03-21-2005, 02:24 PM

> --However, you can always go here ( http://www.namesys.com/ ) and check the FAQ

Jo, that was a good pointer.

However, I found that entry:
"Does using ReiserFS mean I can just press the power off button without running "shutdown" or "init 0," etc? Does it mean there is no risk of data loss?"
(http://www.namesys.com/faq.html#poweroff)
The answer was "no"

and they were using the term "data journaling", what will be introduced in the future.

For xfs it doesn't appear to be any better(?)
(I found http://oss.sgi.com/projects/xfs/faq.html)

**tom p** · 03-21-2005, 07:09 PM

Originally Posted by ThomasX

The answer was "no"

The journaling file systems only guarantee consistency of the file system, not that no data will be lost after a catastrophic failure.

Thomas

**Dave_Bechtel** · 03-21-2005, 08:21 PM

--I think you're a little too paranoid.

--Buy a UPS, and do frequent backups, and you should be fine.

Originally Posted by ThomasX

back to my orig. question: I got an answer:

I assumed that RAID is on top of the fs, but I got told that actually the fs is on top of the RAID module. Therefore there is only *one* fs "instance" involved, which either gets rolled back or not.

However, a new question arises now: If a journaling fs gives savety on power failure, but a raid software is in between, does it probably break your power off savety for the sake of HD HW failure savety?

**ThomasX** · 03-22-2005, 04:04 PM

--I think you're a little too paranoid.

Yes, probably. On the other hand - I really like to hard-switch-off my server, as it is a AT board (no ATX, no soft power button) and has no keyboard attached to it. But o.k., I should change my habits