> Von: ADSM: Dist Stor Manager [mailto:[EMAIL PROTECTED] Auftrag von > Bill Boyer > Gesendet: Donnerstag, 29. April 2004 13:21 > An: [EMAIL PROTECTED] > Betreff: Disk and Tape access over the same fibre adapter > > > Just got a new client that wants to do some LANfree backups, but > they don't > believe me when I tell them that the recommended way is to have a separate > HBA for disk and tape. They don't think that mixing the 2 will be a > performance issue. I remember a couple years ago this was an issue of data > corruption, but that has been addresses. > > Can someone point me to any whitepapers or documentation > explaining why they > should spend a couple more $'s and get an additional adapter for those > server they want to do LANfree backups on? > > TIA, > > Bill Boyer > "Experience is a comb that nature gives us after we go bald." - ??
Hi, don't know anymore where i got this from. I had this discussion some time ago and found the following statement: Mixing disk and tape on a single SCSI bus was rarely done. The workloads are different, the SCSI profiles are different, and it rarely worked satisfactorily. In some instances, tape drives would use large blocks and �data streaming� to maximize performance, tying up the SCSI bus for long periods of time, while disk drives with smaller block sizes appropriate for random I/O would get less than their fair share of the SCSI bus. In other instances, the tape drives would have trouble getting access to the bus because disk drives would respond faster. It was generally accepted that you kept disk and tape on separate SCSI buses. With Fibre Channel, it is possible - in principle - to do better. I/O can be multiplexed and small blocks can be interleaved in the middle of large blocks. There is no shared bus to keep a slow device from sending whenever it is ready. So it is certainly possible with Fibre Channel to have both disk and tape sending or receiving with little interference. There is always the issue of having too much data for the bandwidth available, but this is independent of whether device types are mixed. Unfortunately, to actually take advantage of these capabilities in Fibre Channel, an HBA driver would have to be carefully written to handle this. Sophisticated multithreading would be needed to interleave the I/O fairly. Furthermore, older SCSI drivers were written using a single SCSI profile for the whole adapter. This was fine since disk and tape were rarely, if ever, mixed, so a single profile (one for disk or one for tape) could be used for the whole adapter and work fine. Often this is not under the user�s control - the driver picks a profile based on what devices it sees. HBA drivers could certainly be written to allow different profiles to be used for different devices at the same time on a single adapter, but this is not yet a common practice. Thus, if you do mix disk and tape on a single HBA, you end up either using a disk profile for both disk and tape, or a tape for both disk and tape. Some devices will be using a non-optimal profile. Finally, subtle problems have been discovered whose source has been narrowed down to interference between disk and tape traffic. Typically these problems occur intermittently, but when they do, devices can go offline leading to interruption of service. Also for Performance reason you may consider to have more than one HBA and does not share Disk and Tape I/O on the same HBA. During Backup your server has to read the Data from Disk and write it to the Tape Drive. If it's a LTO 2 Drive you may write more than 35 MB/sec to the Tape Drive. This means you have also to read 35 MB/sec. If those both streams goes over HBA, then this HBA have to handle at least 70 MB/sec. Considering a 1 Gbit HBA you already reached the bandwidth of this HBA. As long as there is no other Disk I/O activity during this backup it may work without any problems. But, if during the Backup still applications running which are accessing the Disk, then you may see impact of Backup performance and impact of the applications performance. For Backupserver, especially TSM Server, where a high I/O load exist you should run with several HBAs and separate the disk and tape IO. For all of these reasons, IBM�s official stance on mixing disk and tape on a single HBA is a little different from other configurations. IBM will support mixing disk and tape on a HBA, and the IBM Support Center will accept problems reported on these configurations. However, if the problem determination process reveals that the problem in question is caused by the mixing of the traffic, IBM may choose to tell the customer that the only fix is to separate the traffic. This normally means installing additional HBA(s), which can be problematic if a server has insufficient adapter slots. Over time, more will be learned about which combinations seem to work satisfactorily, and which combinations have specific problems that will require separation of the traffic. In essence, the customer is assuming some risk in these configurations, and it is strongly recommended to avoid mixing disk and tape traffic on a single HBA whenever possible. // Franco Chiellino // mailto:[EMAIL PROTECTED] >
