Re: Raid 5

It is to laugh. Tell them you want to benchmark the write speed differences (and be sure to compare with RAW devices). Also ask then how they prevent the partial media failure problem? Only EMC tries and that's the DG Clariion technology that I know is not sufficient. That EMC schema is this (and I can't imagine anyone else has come up with a better one): Add a hidden 8 byte CRC code to every disk block (essentially redefine a disk block to be 520 bytes but have the array drivers strip off the CRC code and return only the 512 bytes that the OS expects. This CRC code is supposed to be able to reconstruct larger (up to 32bits IB) corruption per block than the drive's 1 byte CRC code. However, the problem happens LONG before the block ever gets to the firmware that checks the CRC code. When the media begins to fail, the drive detects it and is able to correct it for a while using it's own CRC (or other ECC) coding. When it detects a block that is corrupted, it corrects the data if it can and rewrites the data to a new block reserved for remapping and logically replaces the failed block with the good one. So whenever you ask for block 27 after that you actually get block 12342 or something like that. It does this without reporting ANY errors so the array's firmware never knows that the platters are beginning to become flaky. Also since the data's been corrected before being returned the EMC ECC coding is never triggered. Meanwhile, one block is not all that's trashed, it's just what's been found so far. Eventually the drive will silently replace as many blocks as it can until it runs out of remap blocks, it will then, finally, begin reporting errors to the OS and the array firmware. However, by that time, it's as likely as not that the embedded CRC codes have also been trashed and are useless. IFF the CRC coding were stored independent of the failing drive, MAYBE it would be safer. But, THAT'S what RAID 3, 4, & 5 are trying to do. The problem is that these codes are not checked at read time or write EVER in a RAID5 array! They are ONLY used to reconstruct a failed or manually removed drive. By that time the bad data on one drive has been used to calculate garbage parity on the parity blocks on the remaining drives so the reconstructed blocks of the replacement drive are garbage as well. NO RAID5!!! NO RAID5!!! NO RAID5!!! NO RAID5!!! NO RAID5!!! NO RAID5!!! Art ----- Original Message ----- From: Danny Wright <dwright@sherwoodfoods.com> At: 5/18 16:50 Someone's trying to sell us on RAID 5 and I haven't reviewed Art's objections lately. They're saying all our concerns are no longer valid. Agree or disagree?