Re: [ANNOUNCE]: Generic SCSI Target Mid-level For Linux (SCST), targetdrivers for iSCSI and QLogic Fibre Channel cards released
From: Vladislav Bolkhovitin
Date: Thu Jul 10 2008 - 14:25:31 EST
Nicholas A. Bellinger wrote:
I have only documents, which I referenced. In them, especially
in "2008 Linux Storage & Filesystem Workshop" summary, it doesn't look
as I took it out of context. You put emphasis on "older" vs
"current"/"new", didn't you ;)?
Well, my job was to catch everyone up to speed on the status of the 4
(four) different (insert your favorite SAM capable transport name here)
Linux v2.6 based target projects. With all of the acroynms for the
standards+implementations+linux-kernel being extremly confusing to
anyone who does know all of them by heart. Even those people in the
room, who where fimilar with storage, but not necessarly with target
mode engine design, its hard to follow.
Yes, this is a problem. Even storage experts are not too familiar with
SCSI internals and not willing much to get better familiarity. Hence,
almost nobody really understands for what is all those SCSI processing
in SCST..
BTW, there are another inaccuracies on your slides:
- STGT doesn't support "hardware accelerated traditional iSCSI
(Qlogic)", at least I have not found any signs of it.
<nod>, that is correct. It does it's hardware acceleration generically
using OFA VERBS for hardware that do wire protocol that implements
fabric dependent direct data placement. iSER does this with 504[0-4],
and I don't recall exactly how IB does it. Anyways, the point is that
they use a single interface so that hardware vendors do not have to
implement their own APIs, which are very complex, and usually very buggy
when coming from a company who is trying to get a design into ASIC.
ISER is "iSCSI Extensions for RDMA", while usually under "hardware
accelerated traditional iSCSI" people mean regular hardware iSCSI cards,
like QLogic 4xxx. Hence, your sentence for most people, including
myself, was incorrect and confusing.
But, when I have time for careful look, I'm going to write some LIO
critics. So far, at the first glance:
- It is too iSCSI-centric. ISCSI is a very special transport, so looks
like when you decide to add in LIO drivers for other transports,
especially for parallel SCSI and SAS, you are going to have big troubles
and major redesign.
Not true. Because LIO-Core subsystem API is battle hardened (you could
say it is the 2nd oldest, behind UNH's :), allocating LIO-Core SE tasks
(that then get issued to LIO-Core subsystem plugins) from a SCSI CDB
with sectors+offset for ICF_SCSI_DATA_SG_IO_CDB, or a generically
emulated SCSI control CDB or logic in LIO-Core, or using LIO-Core/PSCSI
to let the underlying hardware do its thing, but still fill in the holes
so that *ANY* SCSI subsystem, including from different OSes, can talk
with storage objects behind LIO-Core when running in initiator mode
amoungst the possible fabrics. Some of the classic examples here are:
*) Because the Solaris 10 SCSI subsystem requiring all iSCSI devices to
have EVPD information, otherwise LUN registration would fail. This
means that suddently struct block_device and struct file need to have
WWN information, which may be DIFFERENT based upon if said object was a
Linux/MD or LVM block device, for example.
*) Every cluster design that required block level shared storage needs
to have at least SAM-2 Reservations.
*) Exporting via LIO-Core Hardware RAID adapters on OSes where
max_sectors cannot be easily changed. This is because some Hardware
RAID requires a smaller struct scsi_device->max_sector to handle smaller
stripe sizes for their arrays.
*) Some adapters in drivers/scsi which are not REAL SCSI devices emulate
none/some WWN or control logic mentioned above. I have had to do a
couple of hacks over the years in LIO-Core/PSCSI to make everything
place nice going to the client side of the cloud, check out
iscsi_target_pscsi.c:pscsi_transport_complete() to see what I mean.
I meant something different: interface between target drivers and SCSI
target core. Here (seems) you are going to have big troubles when you
try to add not-iSCSI transport, like FC, for instance.
And this is a real showstopper for making LIO-Core
the default and the only SCSI target framework. SCST is SCSI-centric,
Well, one needs to understand that LIO-Core subsystem API is more than a
SCSI target framework. Its a generic method of accessing any possible
storage object of the storage stack, and having said engine handle the
hardware restrictions (be they physical or virtual) for the underlying
storage object. It can run as a SCSI engine to real (or emualted) SCSI
hardware from linux/drivers/scsi, but the real strength is that it sits
above the SCSI/BLOCK/FILE layers and uses a single codepath for all
underlying storage objects. For example in the lio-core-2.6.git tree, I
chose the location linux/drivers/lio-core, because LIO-Core uses 'struct
file' from fs/, 'struct block_device' from block/ and struct scsi_device
from drivers/scsi.
SCST and iSCSI-SCST, basically, do the same things, except iSCSI MC/S
and related, + something more, like 1-to-many pass-through and
scst_user, which need a big chunks of code, correct? And they are
together about 2 times smaller:
$ find core-iscsi/svn/trunk/target/target -type f -name "*.[ch]"|xargs wc
59764 163202 1625877 total
+
$ find core-iscsi/svn/trunk/target/include -type f -name "*.[ch]"|xargs
2981 9316 91930 total
=
62745 1717807
vs
$ find svn/trunk/scst -type f -name "*.[ch]"|xargs wc
28327 77878 734625 total
+
$ find svn/trunk/iscsi-scst/kernel -type f -name "*.[ch]"|xargs wc
7857 20394 194693 total
=
36184 929318
Or did I count incorrectly?
Its worth to note that I am still doing the re-org of LIO-Core and
LIO-Target v3.0.0, but this will be coming soon along with the first non
traditional iSCSI packets to run across LIO-Core.
just because there's no way to make *SCSI* target framework not being
SCSI-centric. Nobody blames Linux SCSI (initiator) mid-layer for being
SCSI-centric, correct?
Well, as we have discussed before, the emulation of the SCSI control
path is really a whole different monster, and I am certainly not
interested in having to emulate all of the t10.org standards
myself. :-)
Sure, there optional things. But there are also requirements, which must
be followed. So, this isn't about interested or not, this is about must
do or don't do at all.
- Seems, it's a bit overcomplicated, because it has too many abstract
interfaces where there's not much need it them. Having too many abstract
interfaces makes code analyze a lot more complicated. For comparison,
SCST has only 2 such interfaces: for target drivers and for backstorage
dev handlers. Plus, there is half-abstract interface for memory
allocator (sgv_pool_set_allocator()) to allow scst_user to allocate user
space supplied pages. And they cover all needs.
Well, I have discussed why I think the LIO-Core design (which was more
neccessity at the start) has been able to work with for all kernel
subsystems/storage objects on all architectures for v2.2, v2.4 and v2.6
kernels. I also mention these at the 10,000 ft level in my LSF 08'
pres.
Nobody in the Linux kernel community is interested to have obsolete or
unneeded for the current kernel version code in the kernel, so if you
want LIO core be in the kernel, you will have to make a major cleanup.
Also, see the above LIO vs SCST size comparison. Is the additional code
all about the obsolete/currently unneeded features?
- Pass-through mode (PSCSI) also provides non-enforced 1-to-1
relationship, as it used to be in STGT (now in STGT support for
pass-through mode seems to be removed), which isn't mentioned anywhere.
Please be more specific by what you mean here. Also, note that because
PSCSI is an LIO-Core subsystem plugin, LIO-Core handles the limitations
of the storage object through the LIO-Core subsystem API. This means
that things like (received initiator CDB sectors > LIO-Core storage
object max_sectors) are handled generically by LIO-Core, using a single
set of algoritims for all I/O interaction with Linux storage systems.
These algoritims are also the same for DIFFERENT types of transport
fabrics, both those that expect LIO-Core to allocate memory, OR that
hardware will have preallocated memory and possible restrictions from
the CPU/BUS architecture (take non-cache coherent MIPS for example) of
how the memory gets DMA'ed or PIO'ed down to the packet's intended
storage object.
See here:
http://www.mail-archive.com/linux-scsi@xxxxxxxxxxxxxxx/msg06911.html
- There is some confusion in the code in the function and variable
names between persistent and SAM-2 reservations.
Well, that would be because persistent reservations are not emulated
generally for all of the subsystem plugins just yet. Obviously with
LIO-Core/PSCSI if the underlying hardware supports it, it will work.
What you did (passing reservation commands directly to devices and
nothing more) will work only with a single initiator per device, where
reservations in the majority of cases are not needed at all. With
multiple initiators, as it is in clusters and where reservations are
really needed, it will sooner or later lead to data corruption. See the
referenced above message as well as the whole thread.
The more in fighting between the
leaders in our community, the less the community benefits.
Sure. If my note hurts you, I can remove it. But you should also remove
from your presentation and the summary paper those psychological
arguments to not confuse people.
Its not about removing, it is about updating the page to better reflect
the bigger picture so folks coming to the sight can get the latest
information from last update.
Your suggestions?
Many thanks for your most valuable of time,
--nab
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