RAID 5 slow writes?

[ErikD]

I have a server running Windows SBS 2003. It is a dual PIII 500MHz, 1 GB RAM, 5 SCSI HDs in RAID 5 (American Megatrends Perc 2 SC Card). I noticed an interesting issue when trying to transfer files from my desktop to the server.

A fairly small file (30MB or so) transfers nice and fast. A CD image (500 MB or so) takes a fair amount of time to transfer, about as long as doing a 3GB backup from the server to the desktop. It starts out quick, but then about halfway through the tarnsfer it slowed down.

So what is causing these backups? It can't be the network as I am running 100MBps, and it works fine going one way. I am guessing it might be that I am using RAID 5, and the write speeds for such a large file slow things down as it needs to do the parity and such. I am also thinking that it might be the RAID card having only 16MB of RAM onboard could be the cause.

[bchur83]

The limited cache on the card would hinder performance. Also, I'm not sure about that card, is it a true hardware Raid card with a XOR processor. It sounds to me like it is probably an older card. Also, depending on what interface and type of drives it has could be a bottleneck. Is it a standard 33mhz 32bit PCI card, or 64bit/66mhz?

[ErikD]

It does have the on card XOR processor. I am having some trouble finding the exact specs for the card, and the type of RAM it takes. I know it takes a 72 pin SIMM, but am not sure if it is EDO or FP, Parity or not. I tried some 32MB EDO I have but it would no longer work with that.

I will probably just leave it as is, or see if I can find a manual for the card to confirm the types of compatible RAM.

[dark_15]

I agree with bchur: the low amount of memory is hindering your performance with the larger file transfers. Does THIS (http://america.hongfaith.com/Manual/Express200_spec.pdf) look like your card? If so, I think I know where I can find a manual for you.

[ErikD]

Yeah, that's it. It seems it can take EDO or FPM up to 128MB, but no mention of using parity or non-parity RAM. I will try some other 32MB EDO I have lying around, and see if I can pick up some 128MB sticks. It would just really help if I knew if I needed parit RAM or not.

[ErikD]

I think that it requires parity RAM, as I have a few 32MB non parity sticks around which don't work with it.

How much RAM would I need to jump to in order to see an increase in perfromance? 32MB? 64MB?

[dark_15]

I did find a part number that appears to be compatible with your card...

Take a look HERE (http://www.smartmconfig.com/config/index.asp?MODEL=MegaRAID+Express+200&Submit=GO). I can find it on sale online, but I cannot find any information about the memory other than it is compatible with your controller.

Hope that helps! :)

EDIT: I found a link to some more memory to that card. It looks like it is unbuffered, but parity memory. Hopefully that helps too!

As for how much, it all depends on how much you wish to spend. Even 32mb will yield some sort of performance increase. I would recommend 64mb if you can afford it.

[Snugglebear]

The PERC2 is a rebadged LSI1600, which is the same as the HP Netraid 1SI, of which we have several in the office. They do XORs in software and max out their write speeds at about 8MB/s, regardless of the processor speed (used it briefly with a P4-820, came from a dual P3-667; speed was the same on both machines). For cheap RAID5 they're decent cards, but are sadly outclassed these days by just about everything. Heck, even RAID5 off the ICH7 will clobber it.

The reason you see the server have good performance initially and then have it trail off is that the file transfer is being cached in main memory. For a while this was the primary controller in our main fileserver, and nobody else would believe me until I showed them the performance graphs. The network transfer rate would go along at max speed until the machine's main memory was chewed up, at which point the transfer rate would drop to between 2-5MB/s. If you watch the disk performance you could see the disk continue writing the file out for several minutes after the network transfer was complete.

FYI we were only able to get them to recognize 32MB SIMMs. The 128MB and 64MB SIMMs both failed to detect. The additional cache memory did not boost performance whatsoever. Only replacing the controllers with newer cards did the trick. The MegaRAID 320s (PERC4s) are very nice, and even the 160s are much better.

[ErikD]

Well seeing as how it is just a home server for four users, and almost never using it at the same time, and most tranfers being a Word document or such, I won't be spending that much on it. I found a good deal on some EDO parity in 32MB and 128MB, so I will try that out. If it works, and boosts performance great. If not I can live with it as it is as I almost never transfer such large files anyway.