Generic SCSI Target Middle Level for Linux
+Generic SCSI Target Subsystem for Linux
-The SCSI target mid-level subsystem for Linux (SCST) is an alternative +
The SCSI target subsystem for Linux (SCST) is an alternative implementation of SCSI target subsystem for Linux. It provides unified, consistent interface between SCSI target drivers and Linux kernel as well as between Linux kernel and storage backend diff --git a/www/mc_s.html b/www/mc_s.html index 0123892db..0a84bf763 100644 --- a/www/mc_s.html +++ b/www/mc_s.html @@ -1,7 +1,7 @@
- + @@ -14,7 +14,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
Performance
At first, neither MC/S, nor MPIO can improve performance if there is -only one SCSI command sent to target at time. For instance, in case of tape -backup and restore. Both MC/S and MPIO work on the commands level, so -can't split data transfers for a single command over several links. Only -bonding can improve performance in this case, because it works on the -link level.
+only one SCSI command sent to target at time. For instance, in case of +tape backup and restore. Both MC/S and MPIO work on the commands level, +so can't split data transfers for a single command over several links. +Only bonding (also known as NIC teaming or Link Aggregation) can improve +performance in this case, although also with limitations, because it +works on the link level.MC/S over several links preserves commands execution order, i.e. with it commands executed in the same order as they were submitted. MPIO @@ -172,42 +173,13 @@ commands get executed, although it would be better for performance to reorder them. As result, MPIO sometimes has better performance, than MC/S, especially in setups, where maximum IOPS number is important. See, for instance, - http://article.gmane.org/gmane.linux.scsi/16311.
-Conclusion
+When MC/S is better than MPIO
-Thus:
- --
-
- Benefits of MC/S are marginal and with future MPIO - improvements can be made negligible. - -
- MPIO allows to utilize existing infrastructure for all - transports, not only iSCSI. - - -
- All transports can benefit from improvements in MPIO. - -
- With MPIO there is no need to create multiple layers doing very similar - functionality. - -
- MPIO doesn't have commands ordering bottleneck, which MC/S has. - -
Hence, what's the point to implement MC/S, which is a very tricky -task? Better to spend this effort on improving MPIO. Simply, MC/S is -done on the wrong level. No surprise then that no Open Source OS'es -neither support, nor going to implement it. Moreover, when back to 2005 -there was an attempt to add MC/S in Linux, it was rejected. See for more details -http://article.gmane.org/gmane.linux.scsi/15769 -and http://article.gmane.org/gmane.linux.scsi/16301. -
- -But for sake of completeness, there are marginal cases, where MPIO -can't be used or will not provide any benefit, but MC/S can be successful:
+There are marginal cases, where MPIO can't be used or will not +provide any benefit, but MC/S can be successful:
- When strict commands order is required. @@ -236,8 +208,38 @@ and backup applications one or both can be true. But on practice: +
- Cost to develop MC/S is high, but benefits of it are marginal and with future MPIO + improvements can be made negligible. + +
- MPIO allows to utilize existing infrastructure for all + transports, not only iSCSI. + + +
- All transports can benefit from improvements in MPIO. + +
- With MPIO there is no need to create multiple layers doing very similar + functionality. + +
- MPIO doesn't have commands ordering bottleneck, which MC/S has. + +
- Thread - - "Relationship between SCST and in-kernel SCSI_TGT" - - -
- this - message in thread "Open-FCoE on linux-scsi" for more details. -
Conclusion
+ +Thus:
+ +-
+
Simply, MC/S is done on the wrong level. No surprise then that no +Open Source OS'es neither support, nor going to implement it. Moreover, +when back to 2005 there was an attempt to add MC/S in Linux, it was +rejected. See for more details http://article.gmane.org/gmane.linux.scsi/15769 +and http://article.gmane.org/gmane.linux.scsi/16301. +
+If in future SCSI standards gain possibility to group several I_T nexuses -with possibilities to reassign commands between them and preserve commands +with ability to reassign commands between them as well as preserve commands order among them, the above minor advantages of MC/S over MPIO will be removed and, hence, all investments in it will be voided.
diff --git a/www/scst_admin.html b/www/scst_admin.html index 789eea023..5a7d538f7 100644 --- a/www/scst_admin.html +++ b/www/scst_admin.html @@ -1,7 +1,7 @@ - + @@ -14,7 +14,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
-
@@ -29,7 +29,7 @@
Generic SCSI Target Middle Level for Linux
+Generic SCSI Target Subsystem for Linux
Vladislav Bolkhovitin
Version 0.9.3-pre4 2006/02/07, actual for SCST 0.9.3-pre4 and later
This document describes SCSI target mid-level for Linux (SCST), its architecture and drivers from diff --git a/www/scstvsstgt.html b/www/scstvsstgt.html index f13bf614b..b7d46ce53 100644 --- a/www/scstvsstgt.html +++ b/www/scstvsstgt.html @@ -1,7 +1,7 @@
- + @@ -14,7 +14,7 @@diff --git a/www/sponsorship.html b/www/sponsorship.html index 1ac4286ee..6bd40350e 100644 --- a/www/sponsorship.html +++ b/www/sponsorship.html @@ -12,7 +12,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
@@ -44,7 +44,7 @@STGT is alternative, independent from SCST implementation of SCSI target framework for Linux. It has different architecture, where SCSI target state machine is placed in the user space, while in SCST all the processing done in the kernel. Such architecture as STGT has was acknowledged - at the moment by the Linux SCSI subsystem maintainers as a "right" one, so kernel's part of STGT quickly + by the Linux SCSI subsystem maintainers as a "right" one, so kernel's part of STGT quickly found its way to the kernel.
But such architecture has several inherent problems. Among them performance and complexity. @@ -66,30 +66,12 @@ http://article.gmane.org/gmane.linux.scsi/21073 why STGT's architecture was considered "right". -
Also you shouldn't be deceived by the fact that some (small) part of STGT was accepted into the kernel.
- -
It doesn't mean that STGT has the "kernel quality". In fact, STGT as a whole similarly to any other out-of-tree project lives on its own, hence has its own quality level, which isn't necessary better, than the - quality level of SCST. Actually, from such important aspect of quality as simplicity, it - might be quite contrary: e.g. SCST isn't required to support HIGHMEM (nowadays it - isn't necessary, but required for all in-kernel components), which allowed to simplify memory management a lot.Interesting, on 2008 Linux Storage & Filesystem Workshop, namely in - "Storage Track" and - "Linux/iSCSI and a Generic Target Mode Storage - Engine for Linux v2.6" documents, a special emphasis was put on the fact that SCST has "older" design, - while STGT has the "current" design. Well, this seems to be a good sign, because SCST opponents have come to so - little to say against it, so they started to use psychological arguments, exploiting the fact that most people - think that "newer" is always better.
+ quality level of SCST.-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
diff --git a/www/target_fcoe.html b/www/target_fcoe.html index dd3186701..60126a1fd 100644 --- a/www/target_fcoe.html +++ b/www/target_fcoe.html @@ -1,7 +1,7 @@ - + @@ -14,7 +14,7 @@
-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
diff --git a/www/target_iscsi.html b/www/target_iscsi.html index c78216215..be692d5b0 100644 --- a/www/target_iscsi.html +++ b/www/target_iscsi.html @@ -1,7 +1,7 @@ - + @@ -14,7 +14,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
@@ -107,10 +107,8 @@Also, in contrast to IET, iSCSI-SCST is open for any new development, modifications and - improvements, so people who want to fix or implement something new will not have to keep and maintain separate - patches as it is currently necessary with IET. ISCSI-SCST is actively - developed and going to gain in the future new features like support for multiple connections per session, error recovery - levels >0, etc., which eventually make it really "Enterprise".
+ improvements, so people who want to fix or implement something new will not have to keep and maintain separate + patches as it is currently necessary with IET.If you are an IET user before installation carefully read README files of both iSCSI-SCST and the SCST core. Especially pay attention that now the LUN information for iSCSI-SCST is configured not using diff --git a/www/target_local.html b/www/target_local.html index 851331f51..fdea997e6 100644 --- a/www/target_local.html +++ b/www/target_local.html @@ -1,7 +1,7 @@
- + @@ -14,7 +14,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
diff --git a/www/target_lsi.html b/www/target_lsi.html index a2c95cd2c..4a03cab64 100644 --- a/www/target_lsi.html +++ b/www/target_lsi.html @@ -1,7 +1,7 @@ - + @@ -14,7 +14,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
diff --git a/www/target_mvsas.html b/www/target_mvsas.html index 11c2193af..e918acf78 100644 --- a/www/target_mvsas.html +++ b/www/target_mvsas.html @@ -1,7 +1,7 @@ - + @@ -14,7 +14,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
diff --git a/www/target_old.html b/www/target_old.html index da1cafcef..d942d1af3 100644 --- a/www/target_old.html +++ b/www/target_old.html @@ -1,7 +1,7 @@ - + @@ -14,7 +14,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
diff --git a/www/target_qla22xx_23xx.html b/www/target_qla22xx_23xx.html index a908f6c5c..d4078ff76 100644 --- a/www/target_qla22xx_23xx.html +++ b/www/target_qla22xx_23xx.html @@ -1,7 +1,7 @@ - + @@ -14,7 +14,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
diff --git a/www/target_qla_isp.html b/www/target_qla_isp.html index 6006256ee..6cd4258cd 100644 --- a/www/target_qla_isp.html +++ b/www/target_qla_isp.html @@ -1,7 +1,7 @@ - + @@ -14,7 +14,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
diff --git a/www/target_srp.html b/www/target_srp.html index a2d9a3fc6..50cd6bf2e 100644 --- a/www/target_srp.html +++ b/www/target_srp.html @@ -1,7 +1,7 @@ - + @@ -14,7 +14,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux
diff --git a/www/targets.html b/www/targets.html index 12d9772ac..18eaf655c 100644 --- a/www/targets.html +++ b/www/targets.html @@ -1,7 +1,7 @@ - + @@ -14,7 +14,7 @@-
SCSI Target Middle Level for Linux
+SCSI Target Subsystem for Linux