[LU-16938] "lfs setstripe -C -1" stripes too widely, should be limited to OST_COUNT

• Type: Improvement
• Resolution: Fixed
• Priority: Major
• Fix Version/s: Lustre 2.16.0
• Affects Version/s: Lustre 2.15.0
• Severity: 3
• Assignee: Rajeev Mishra
• Reporter: Rajeev Mishra

Description

I am reaching out to seek clarification regarding the expected behavior of the "lfs setstripe" command when using the -C -1 option.

Currently, it appears that this command is creating a higher stripe count than anticipated. For instance, on my test system, it generated a stripe count of 2727 for a single file. This count exceeds the allowed limit of LOV_MAX_STRIPE_COUNT. 

I am uncertain about the appropriate solution to address this issue related to the "-1" argument. I have contemplated the following options:

1.    Consider making the option -1 illegal, preventing its usage altogether.

2.    Implement a mechanism to automatically set the stripe count to the maximum allowed value (LOV_MAX_STRIPE_COUNT) if the count exceeds this limit.

I would greatly appreciate your input and guidance in this matter. It is worth noting that setting the stripe count higher than LOV_MAX_STRIPE_COUNT leads to other problems, such as the failure of the "llapi_layout_get_by_fd" API to open the file.

Please let me know your input.

Issue Links

• is related to: LU-17925 sanity-flr test_0b: lfs mirror create: cannot create composite file: Numerical result out of range
• is related to: LU-17199 'lfs setstripe -C -1' can be set beyond overstripe count > 2000
• is related to: LU-18244 add 'lfs mkdir -C -N' overstriping support
• is related to: LU-16623 lod_statfs_and_check() does not skip unusable OSTs
• is related to: LU-13748 'lfs setstripe -C -1' stripes too widely

Comments

[Patrick Farrell]

Josh,

Makes sense to me.  There's also another possible bug here - how many OSTs do you have on that system?  If it's >= 10, then overstriped shouldn't be set by the server code either, which is also a concern.  Overstriping should only be set on the file when the actual file striping exceeds the number of available OSTs.  (Or at least that was the intent...)

So there may be two things to fix there - proper overriding by later parameters in userspace, so the overstriping flag isn't passed along, and then - if you have >= 10 OSTs, then the server shouldn't set the overstriping pattern regardless of what userspace asked for.  If you have 20 OSTs and give -C 10, overstriping shouldn't be set, because the file is not actually overstriped.  Overstriping set on a not-overstriped file isn't fatal, but it's definitely wrong.

[Josh Schwartz]

> Like many utilities, the last option specified will take precedence.

I would be fine with either (mutually exclusive or last takes precedence in its entirety) but this bothers me:

jupiter-p2:/lus/kjcf08 # mkdir test
jupiter-p2:/lus/kjcf08 # lfs setstripe --overstripe-count 1024 --stripe-count 10 test
jupiter-p2:/lus/kjcf08 # lfs getstripe -d test
test
stripe_count:  10 stripe_size:   1048576 pattern:       raid0,overstriped stripe_offset: -1

Note that we got (and kept) overstriped from the first param, but then picked up the count from the second. If the last option truly took precedence I would expect a stripe count of 10 without overstriped (just like if the first one took precedence I would expect a stripe count of 1024 with overstriped).

It is inconsistent that the behavior is different if you issue them individually, but in the same order:

jupiter-p2:/lus/kjcf08 # lfs setstripe --overstripe-count 1024 test
jupiter-p2:/lus/kjcf08 # lfs getstripe -d test
stripe_count:  1024 stripe_size:   1048576 pattern:       raid0,overstriped stripe_offset: -1
jupiter-p2:/lus/kjcf08 # lfs setstripe --stripe-count 10 test
jupiter-p2:/lus/kjcf08 # lfs getstripe -d test
stripe_count:  10 stripe_size:   1048576 pattern:       raid0 stripe_offset: -1

Here each command does as I would expect; --overstripe-count 1024 by itself yields overstriped and stripe count 1024, and --stripe-count 10 by itself on the same directory removes overstriped (which is what I would expect) yielding stripe count 10 without overstriped.

The fact that combining them causes it to take the overstriped from the first param and the stripe count from the second is surprising. --stripe-count explicitly means not-overstriped and if the rule is that the last one takes precedence, then it should be like the --overstripe-count wasn't there at all instead of the --stripe-count acting as a modifier.

[Patrick Farrell]

Interesting, OK!  Happy to defer.  I wasn't familiar with "last option takes precedence".

[Andreas Dilger]

Actually, I will disagree with Patrick here. While "-C M" enables overstriping, it works with a stripe count that is less than the number of OSTs < M, equivalent to "-c M" in that case. I don't think it "conflicts" with a later "-c N" option. Like many utilities, the last option specified will take precedence.

[Patrick Farrell]

It would be OK to fix that as a separate patch on this ticket, it's small enough it doesn't need a new ticket.  But it should be a separate patch, that's all.

[Rajeev Mishra]

Ok I will just take care of n*ostcount as part of this ticket. Thanks Patrick

[Patrick Farrell]

Interesting!  I think we should make them mutually exclusive, yes.  I would do that in a separate patch from the one which adds '-2, -3' functionality.

[Rajeev Mishra]

I'm currently reviewing the options -c, -C, overstripe-count, and stripe-count to gain a better understanding of their behavior.

I've noticed that there's a bug where the command accepts all of these options simultaneously, even though it appears that they should be mutually exclusive. Presently, all flags can be used together, as demonstrated in the following example:

[root@test2-rocky8 jbs]# lfs setstripe --overstripe-count 1024 --stripe-count -1 -c 10 -C -1 test

[root@test2-rocky8 jbs]# lfs getstripe test | more

test

lmm_stripe_count:  2000

lmm_stripe_size:   1048576

lmm_pattern:       raid0,overstriped

lmm_layout_gen:    0

 

The documentation for these options are as follows:

 

    -c, --stripe-count <stripe_count>: Specifies the nu    mber of OSTs to stripe a file over. A value of 0 means to use the filesystem-wide default stripe count (default is 1), and -1 means to stripe over all available OSTs.

 

    -C, --overstripe-count <stripe_count>: Specifies the number of stripes to create, creating more than one stripe per OST if the count exceeds the number of OSTs in the file system. Similar to -c, 0 uses the filesystem-wide default stripe count (default is 1), and -1 means to stripe over all available OSTs.

    

Now, the question arises: should we consider making these options mutually exclusive? The reason for this consideration is that the behavior of -c is essentially the same as using -C,. If we allow mutual inclusion, we would need to define the precedence of these options when used together.

Your feedback and input on whether or not we should make these options mutually exclusive would be greatly appreciated.

The fix will stick with the pattern mentioned in the comment above, which means it will use -1, -2, -3, and so on. In simpler terms, the stripe count will be calculated as a multiple of the OST count, up to the maximum stripe count allowed.

*~ *                                                                                                     

[Andreas Dilger]

Since the core "-C -1" issues were already fixed by LU-13748 and LU-16623, I changed this issue to track the improvement for mapping "-C -1" to use OST_COUNT like "-c -1", and "-C -2" to use "2 * OST_COUNT", etc.

[Patrick Farrell]

Definitely not.  That can cause crashes, or at least errors (or it should).

[Rajeev Mishra]

@Andreas I will try to add the functionality as suggested. I assume max in any case should not cross LOV_MAX_STRIPE_COUNT that is 2000 ?