VERY slow performance on a DELL PowerEdge R210 II with H200 raid controller when i enable raid

[stanastasios]

hello,

my current configuration is

PowerEdge R210 II

perc H200 controller

2 SAS 1TB HDD

8GB RAM

I have tried to install esxi 5.5 from vmware and from DELL custom ISO but the results are the same. Very slow speed when i have RAID enabled (RAID 1).

I also atached to my message a photo from disk performance. Is there anyone with the same problem ?

[a_p_]

Welcome to the Community,

according to Dell PowerEdge RAID Controller (PERC) | Dell the H200 controller doesn't support battery-backed cache. Without this option the controller will operate in write-trough mode (rather than write-back with cache), i.e. each write operation needs to be acknowledged by the disks, thus the high latency. ESXi doesn't do any write caching itself for data safety reasons and fully relies on the RAID controller.

André

[stanastasios]

I am aware of this problem but the extreme delay is only due to lack of battery ? speed in the graph that i post is reasonable ? Is there any place so that i can undrestand the difference between a raid controller with/without battery support ?

[stanastasios]

It took me 1hour to install ms sql server 2012 express !

[a_p_]

From my personal experience, the difference between write-through and write-back operation is usually >10x. In numbers, I saw 5-10MB/s without, and >100MB/s with write-caching.

André

[stanastasios]

i will buy a new controller and i will post my results. thank you very much !

[JarryG]

"...I am aware of this problem but the extreme delay is only due to lack of battery ?..."

No, it is not because of missing battery, but because of missing CACHE, which (according to Dell's web-site) H200 simply does not have:

http://www.dell.com/learn/us/en/04/campaigns/dell-raid-controllers

Battery would be completely useless with this adapter, as it has nothing to back up with power. So buy controller with on-board cache (the bigger the better). On most controllers you can overwrite default settings and use cache for writing even without battery, but I would not recommend it (unless you have UPS). So I'd recommend to buy controller with cache AND battery-backup unit (or super-capacitor)...

[a_p_]

That's certainly true, if there's no cache, there's nothing to back up

Btw. newer controllers usually use flash backed cache rather then battery backed cache (as shown in the link I posted earlier).

André

[JarryG]

flash-backed cache is not solution for everything. If you are using flash-memory for storage (i.e. SSD), it would not make much sense to cache data to flash, if you want to write it to flash. The speed of cache and storage would be about the same. Flash-backed cache is more like "level-2 cache": bigger, but slower than ddr-based cache. Still very usefull with common hard-drives, but not so much effective with SSD, or hybrid-raid (HDD+SSD). And not to forget, number of re-writes of flash-based memory is rather limited.

Newer LSI-controllers (and their re-branded derivatives) still use conventional very fast ddr-chips for caching (much faster then the fastest flash-memory), but backed by "super-capacitor" (big capacitor) instead of battery. The same cache-protection is used internally by enterprise-targeted SSDs (which have quite big cache too)...

[a_p_]

I'm actually not talking about flash-based cache, but about flash-backed cache, where the content of the cache is backed up to flash memory in case the host goes down. This way the controller only needs a small capacitor with enough power to backup the data to flash, rather than a high-capacity battery to hold the memory in the cache RAM.

André