Le Fri, 22 Jan 2016 02:10:03 +0000 "Seymour, Shane M" <shane.seymour@xxxxxxx> écrivait: > > However, before making the final patch, we should decide which > > partition the specified size should apply to. For the SCSI level > > <=2 it applies to partition 1. For other drives we may have some > > freedom to “tune” the definition. The size should apply to the > > partition the users expect it to apply. > > I'd argue for consistency here in the current interface and that it > should apply in the same way more on that below. I agree, it would be extremely surprising (in a bad way) to see your application behave differently when upgrading your drive, for instance. > > Partitioning with two partitions is used for storing index in a > > small partition and use the rest of the tape for data. In this > > case, it is probably natural to specify the size of the index. The > > LTFS definition supports index in any partition. The open source > > code I have seen seem to default to index in partition 0. > > > > The HP and IBM LTO default partitioning (FDP=1) specifies two wraps > > (minimum) to partition 1 and the rest to 0. LTFS 2.2 specifications doesn't explicitly number partitions, however it makes it clear that the "index" partition (the smaller one) must come first. Furthermore, all LTFS tapes I've had a look at have partition 0 as index. Add to this the fact that current LTO-7 tapes are 6 TB. That comes to pretty big numbers when you want to size a partition in MB. > It may be worth noting (if you're going to update any documentation) > that isn't 100% accurate. You actually get one wrap in partition 1 > and the rest minus one wrap into partition 0. There is one wrap used > as a guard between the two partitions. The size given to a partition > is rounded up to the size of a wrap as well. > > See > https://docs.oracle.com/cd/E21419_04/en/LTO5_Vol3_E5b/LTO5_Vol3_E5b.pdf > > Page 100 where it gives examples of how partition sizes are > calculated on HP LTO5 drives. > Ican confirm this from the tests I've made on LTO-5 and LTO-6. The size of the partition you'll obtain may end quite different from the size you've asked for. This is made particularly difficult when you must convert a few TB to MB, and you don't know exactly where it'll > > > > There seem to be lot of arguments supporting both possible choices. > > Should we use the existing definition (1) or change it for the > > drives supporting SCSI level >= 3 (or supporting FORMAT MEDIUM)? > > The definition can’t be changed later. This is why we should make a > > good decision. > > > > Opinions? > > How about using the fact the size is signed to indicate one slightly > different thing? I'm not sure if you'd call this using or abusing the > fact that it's signed. > > Make the default behavior for a positive size the same as the current > behavior for SCSI-2 (size applies to partition 1). If the size is > negative then the absolute value of the size applies to partition 0. > That provides some flexibility in choosing which partition the size > applies to if it worked that way for all devices. > > With that you also get consistent behavior between tape drives without > having to know if the size will apply to partition 0 or partition 1 > based on the tape technology and you get the benefit of being able to > set an explicit size for partition 0 or partition 1. > > You could overload the value of 0 as well to use FDP to choose the > sizes for the partitions (assuming 0 doesn't already have a side > effect in the code). Then you get whatever the tape drive wants to do. -- ------------------------------------------------------------------------ Emmanuel Florac | Direction technique | Intellique | <eflorac@xxxxxxxxxxxxxx> | +33 1 78 94 84 02 ------------------------------------------------------------------------ -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
