|From:||Vladislav Bolkhovitin <firstname.lastname@example.org>|
|Subject:||[PATCH][RFC 0/0/0/5] New SCSI target framework (SCST) with dev handlers and 2 target drivers|
|Date:||Tue, 13 Apr 2010 17:03:55 +0400|
|Cc:||email@example.com, scst-devel <firstname.lastname@example.org>, James Bottomley <James.Bottomley@HansenPartnership.com>, Andrew Morton <email@example.com>, FUJITA Tomonori <firstname.lastname@example.org>, Mike Christie <email@example.com>, Jeff Garzik <firstname.lastname@example.org>, Linus Torvalds <email@example.com>, Vu Pham <firstname.lastname@example.org>, Bart Van Assche <email@example.com>, James Smart <James.Smart@Emulex.Com>, Joe Eykholt <firstname.lastname@example.org>, Andy Yan <email@example.com>, firstname.lastname@example.org, Jens Axboe <email@example.com>, Daniel Henrique Debonzi <firstname.lastname@example.org>|
Please review this second iteration of the patch set of the new (although, perhaps, the oldest) SCSI target framework for Linux SCST with a set of dev handlers and 2 target drivers: for iSCSI (iscsi-scst) and for Infiniband SRP (srpt). The first iteration you can found here: http://lkml.org/lkml/2008/12/10/245. Please review this patchset as a proposed replacement of the current mainline SCSI target subsystem STGT. I've already described advantages of SCST over STGT in http://lkml.org/lkml/2008/12/10/245. In short, they are: 1. Performance, including various performance improvements not available from user space, for instance, because of the user space allocated memory. 2. Overall simplicity with the resulting simpler and more clear code, because STGT has a microkernel-like architecture, but SCST has the same monolithic architecture as the Linux kernel has chosen from the very beginning. 3. Complete pass-through support, which isn't practically possible if the SCSI target core stays in user space. I can add to what I already wrote only: 1. There are recent performance comparison data between SCST SRP and STGT iSER measured by Bart Van Assche with the following target setup: * 22.214.171.124 kernel with SCST patches and with kernel debugging disabled. * OFED 1.5 IB drivers. * SCST revision 1504 with FILEIO vdisk, built in release mode (make debug2release) and with SCST_MAX_TGT_DEV_COMMANDS changed from 48 into 256. * ib_srpt kernel module parameters thread=0. * STGT revision 1.0.1 with rdwr backend. * 1 GB file residing on a tmpfs filesystem was exported towards the initiator system. * Frequency scaling was disabled. * Runlevel: 3. * IRQ affinity for mlx4-comp-0: not bound to a core (smp_affinity=3). * IB HCA: QDR (40Gbps) Mellanox ConnectX MT26428 * CPU: Intel Core2 Duo E8400 @ 3.00 GHz. * NOOP I/O scheduler and initiator setup: * Vanilla 2.6.33-rc7 kernel * SRP initiator was loaded with parameter srp_sg_tablesize=128 * Frequency scaling was disabled. * Runlevel: 3. * IRQ affinity for mlx4_core: bound to a single core (smp_affinity=1). * IB HCA: QDR (40Gbps) Mellanox ConnectX MT26428 * CPU: Intel Core2 Duo E6750 @ 2.66 GHz. The test application was dd utility in O_DIRECT mode run 3 times, then average was calculated. Caches were dropped between each run. For dd bs 4KB: SCST: write 84 MB/s, read 104 MB/s STGT: write 62 MB/s, read 64 MB/s For dd bs 6MB: SCST: write 1030 MB/s, read 2944 MB/s STGT: write 796 MB/s, read 1702 MB/s I've chosen those values for dd bs, because they allow to measure 2 fundamental properties of any link: latency (with bs 4KB) and bandwidth (with bs 6MB). You can see that SCST up to 63% better in latency and up to 73% better in bandwidth! Here is clearly seen the user space implementation overhead! Are there any other evidences needed? 2. Since the first SCST patches review iteration in December 2008 popularity of STGT, despite of the being "mainline", has not grown noticeably, while popularity of SCST has significantly grown. Particularly, Emulex and Marvell added SCST target drivers for their hardware (thanks a lot!), Joe Eykholt added FCoE target (thanks a lot too!) as well as many storage companies are now either selling SCST-based storage devices (see http://scst.sourceforge.net/users.html), or preparing to sell them (so not yet listed on the users page). STGT was originally introduced in 2005 as a "simpler" SCST, where the SCSI target state machine moved from the kernel to user space with goal to create smaller in-kernel code in a hope that it would be similarly effective as the fully in-kernel approach SCST using, but would create less the in-kernel part's maintaining effort. Now, after nearly 5 years passed, it is clear that the overhead of the split kernel/user processing of STGT is much higher than with the fully in-kernel processing of SCST. Thus, we can see now that the hopes for the similarly effective processing of STGT were not correct. We can also see now that the size of in-kernel part only doesn't matter without considering the overall size of the system including the user space part (see http://lkml.org/lkml/2007/4/24/364). Thus, there are no points now left to keep STGT in the kernel. Usually, if for the kernel there are more than one patch/product/etc. doing the same functionality, users are allowed choose the best one by voting for it by using it. So, from this point it is also clear that users have been voting for SCST, not STGT (see above). Just, for instance, count the number of target drivers for SCST: for QLogic (Fibre Channel), Emulex (FC and FCoE), Marvell (SAS) and LSI (parallel SCSI, FC and SAS) hardware as well as for iSCSI, SRP (InfiniBand) and FCoE! While STGT has target drivers only for iSCSI/iSER and IBM pSeries Virtual SCSI (ibmvstgt). Moreover, (Open)Solaris is now developing similar to SCST fully in-kernel SCSI target subsystem COMSTAR. Solaris developers are similarly started from the user space approach, but quickly realized its limitations and moved to the fully in-kernel approach. Thus, we believe, that 5 years is sufficient time to decide that the original hopes for STGT were not correct, STGT is worse than SCST, and users are voting for SCST, therefore it is a time for Linux kernel to acknowledge those and choose the best option. While Linux is loosing time with the worse approach, COMSTAR is going much ahead. Currently, the kernel has only one target driver for STGT: ibmvstgt from drivers/scsi/ibmvscsi, so this is the only driver that would be affected by the removal of the in-kernel part of STGT. STGT iSCSI/iSER target will not be affected, because it's implemented fully in user space and doesn't use any services of the in-kernel part of STGT. Regarding ibmvstgt, we don't know how many users this driver has, but I guess, only few at best, because it is for very special IBM's mainframe virtualization hardware (is it still produced?) and I wasn't able to find maintainer for it in the MAINTAINERS file for 2.6.33. Anyway, we are willing to do the best to migrate this driver to SCST. But who is the maintainer who we should contact? Without hardware we can make at the best a compile tested only version. In future, as I wrote in http://lkml.org/lkml/2008/12/10/245, the user space part of STGT could be a good supplement for SCST as a framework to produce SCST user space targets via scst_local module (see http://lkml.org/lkml/2008/12/10/289), although so far I have not seen any interest to development of user space target drivers. Since the first iteration of the SCST patches, together with a lot of other new features and improvements (version 2.0 is going to be released soon), we have fixed all review comments and added to SCST a sysfs-based interface instead of the old not allowed procfs-based interface. Also we reduced amount of the kernel patches touching the kernel's code outside of SCST and its drivers. The the new sysfs interface is nice looking and easy to use. It is a big step ahead. Detail description of it with a sample layout you can find in the SCST docs. The exceptional feature of the new sysfs interface is that it is self-documented, i.e. with it for any management utilities, like scstadmin , there's no need to know anymore how to configure each specific target driver and dev handler. In other words, the management code will be made once and will work for all current and future targets and dev handlers, including implemented both in kernel and user spaces, without any internal changes. To achieve that all is necessary is that all target drivers and dev handlers should follow few several simple rules how to represent their internal configuration on the sysfs. You can find the sysfs rules also in the SCST doc patch. Any comments are welcome. This iteration for simplicity contains only 2 target drivers: for iSCSI and SRP. If SCST accepted, we will submit other mainline ready drivers later: for QLogic, Emulex and Marvell hardware + scst_local +, probably, FCoE target fcst, if Joe Eykholt thinks it's ready. This patchset is for kernel 2.6.33. In the next iteration, if we don't be told during this review anything really bad, in few weeks time we are going to prepare a request for inclusion patch set. Home page of SCST is http://scst.sourceforge.net Home page of iSCSI-SCST is http://iscsi-scst.sourceforge.net Home page of SCST SRP target driver is http://scst.sourceforge.net/target_srp.html Thank you for your time, Vlad  Scstadmin is an utility, which allows doing SCST configuration using a text config file. Among other, it has the following great facilities: 1. A possibility to apply changes in the config file to currently running system. Only changes applied, so there are no any unneeded restarts and resets. 2. Generate a config file for currently running system. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to email@example.com More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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