On the ESS the write cache is not optional. You must use it. That is where all the write performance comes from. It eliminates the RAID-5 Write penalty and basically changes the writes to RAID-3, no reads before write, when writing sequentially.
Not sure what you mean by 2 write cycles. RAID-1 has two write cycles. RAID-5 has one write cycle to all the drives in the stripe set unless all the parity is on a single drive, but no one does it that way anymore (EMC kind of does with RAID-S). The issue with RAID-5 is the read before write so that you can recalculate the parity. On sequential write of full internal 32K blocks (6 or 7 depending on the disk group) you do not need to read back because you are going to write the whole stripe. So, the ESS calculates the parity in the SSA adapters and writes it to the disks as the IO occurs. The rest of the stuff in here is really good stuff. Though backupsets are extremely hard to manage for customers with only 3494/3590 libaries because the client cannot do the restore. The reality is they run so fast anyway many of the issues that backupsets eliminate do not necessarily apply if you use collocation and parallel restores of them. -----Original Message----- From: Don France (P.A.C.E.) [mailto:[EMAIL PROTECTED]] Sent: Saturday, March 16, 2002 6:40 PM To: [EMAIL PROTECTED] Subject: Re. War stories: Restores > 200GB ? There are several keys to speed in restoring a large number files with TSM; they are: 1.. If using WindowsNT/2000 or AIX, be sure to use DIRMC, storing primary pool on disk, migrate to FILE on disk, then copy-pool both (this avoids tape mounts for the directories not stored in TSM db due to ACL's); I've seen *two* centralized ways to implement DIRMC -- (1) using client-option-set, or (2) establish the DIRMC management class as the one with the longest retention (in each affected policy domain); 2.. Restore the directories first, using -DIRSONLY (this minimizes NTFS db-insert thrashing); 3.. Consider multiple, parallel restores of high-level directories -- despite potential contention for tapes in common, you want to keep the data flowing on at least one session to maximize restore speed; 4.. Consider using CLASSIC restore, rather than no-query restore -- this will minimize tape mounts, as classic restore analyzes which files to request and has the server sort the tapes needed -- though tape mounts may not be an issue with your high-performance configuration; 5.. If you must use RAID-5, realize that you will spend TWO write cycles for every write; if using EMC RAID-S (or ESS), you may want to increase write-cache to as large as allowed (or turn it off, altogether). Using 9 or 15 physical disks will help. A client of mine just had a server disk failure last weekend; it had local disk configured with RAID-5 (hardware RAID controller attached to Dell-Win2000 server) -- after addressing items 1 to 3, above, we were able to saturate the 100Mbps network, achieving 10-15 GB/Hr for the entire restore -- only delays incurred were attributable to tape mounts... this customer had an over-committed silo, so tapes not in silo had to be checked-in on demand. 316 GB restored in approx. 30 hours. Their data was stored under 10 high-level directories, so we ran two restore sessions in parallel -- only had two tape drives -- and disabled other client schedules during this exercise. For your situation, 250 GB and millions of files, and assuming DIRMC (item #1, above), you should be able to see 5 - 10 GB/Hr -- 50 hours at 5 GB/Hr, 25 hours at 10 GB/Hr. So you are looking at two or three days, typically. Large numbers of small files is the "Achilles Heal" of any file-based backup/restore operation -- restore is the slowest (since you are fighting with the file system of the client OS) because of the way file systems traverse directories and reorganize branches "on the fly", it's important to minimize the "re-org" processing (in NTFS, by populating the branches with leaves AFTER first creating all the branches). We did some benchmarks and compared notes with IBM; on another client, we developed the basic expectation that 2-7 GB/Hr was the "standard" for comparison purposes -- you can exceed that number by observing the first 3 recommended configuration items, above. How to mitigate this: (a) use image backup (now available for Unix, soon to be available on Win2000) in concert with file-level progressive incremental; and (b) limit your file server file systems to either 100 GB or "X" million files, then start a separate file system or server upon reaching that threshold... You need to test for your environment to determine what is the acceptable standard to implement. Hope this helps. Don France Technical Architect - Tivoli Certified Consultant Professional Association of Contract Employees (P.A.C.E.) San Jose, CA
