[UR-WG] Draft/suggestion for Storage Accounting Record (2010/10/07)

[Henrik Thostrup Jensen]

Hi

(this is send both to the ogf ur list and the emi sar list, sorry for 
duplicates).

Jon and me happened to be at the same meeting this week, so we took out a
couple of hours of the programme to create a draft suggestion for a storage
accounting record. The suggestion is mainly meant as a way to kickoff the
meeting in Brussels.


1. Discrete vs continuous

First we discussed how storage differs from jobs in the sense that a job is a
discrete unit whereas storage has a more continuous nature, where the usage more
or less constantly varies (bytes used), but typically in relatively small
scale.

A storage record can however only describe something discrete, so the
continuous nature of storage will have to be split into something discrete
(similar to integration). Of course the granularity of this process should not
be dictated be the standard. The first suggestions in then to have a start and
an end time for the storage measurement where a measurement is taken. This
allows to create per-day, per-hour resolution or something third, depending on
the needs.

Result:
"StartTime" and "EndTime" element for describing the time interval for when the
record is valid. Both of them are DateTime values.


2. What to measure

The most obvious metric here is the amount of used space. Another metric is the
amount of reserved space, i.e., space not taken by actual files/objects, but
which cannot be used by other parties. A third metric, is the amount of space
which is not allocated, and can be used. Initially we called this "free space",
but this term is not exactly accurate as it refers more to a file system
metric, which can be very different from what can actually be used due to
quotas. Instead we choose to have an element describing how much unallocated
space there is, i.e., how space one can be expected to be able to use. The
actual free space is a metric, which can be very tricky to measure, and is
typically only of interest to the storage system administrator.

A secondary issue is how to report the numbers. We quickly decided on using
bytes, as it is the fundamental unit, and saves us from deciding on weather to
use 1000 or 1024 bytes as a base. Having multiple options (say KB, MB) for
reporting would complicate the standard unnecessarily without any real benefits,
so we suggest to keep it to bytes, and bytes only. This would also ensure that
the number is always an integer (when using KB and up, floats could be used to
be exact) and saves silly conversion routines when parsing the record.

We briefly considered if reporting should be per file, but this was quite
quickly shoot down, as it would make the records unreasonably large, without
providing any real value. We did end with an element describing the number of
files, which are using the space reported.

Result:
"UsedSpace", "ReservedSpace", "UnAllocatedSpace" metrics for describing how
space is used, reserved, and available for use. Reserved and unallocated are
probably not overlapping (as reserved space is technically used). The
measurement is in bytes.
"FileCount" for describing the number of files using the space.


3. Site and Storage Concepts

The issue of how to describe the site and what storage the accounted data is
stored is perhaps the most complex issue in defining this format. The
discussion to achieve this was very non-linear, so I will just the describe the
result:

A site is considered a top level container for storage. The site name should
globally unique (which probably means an FQDN).
A storage system is an independent system on a site.
A storage system partition is a part of storage system (similar to dCache pools)
A storage type describes the storage type where the data is stored (disk, tape)

None of these elements are mandatory (though we couldn't find a valid use case
for a record without a site). E.g., is perfectly valid to just use site, site
and storage type, or site and storage system. If multiple of the elements
exist, they are considered to have hierarchical tree-like structure, i.e.,
site -> storage system -> storage system partition -> storage type. How to
structure this is described in the "Record Structure" section.

This allows a site with a simple setup to reporting just for the site, where as
more complex installations can report per storage system, and how much is
placed on tape and disk respectively for different storage systems.

A single institution can easily report multiple sites, we do not interfere in
this, but it is very likely that a site would have several storage element /
systems and would like to able to aggregate it under a single site.

Finally we also add the possibility for a storage class, which describes the
class of storage accounted for. This could "precious", "deletable", "pinned",
etc. It is not considered a part of the hierarchy described above.


4. Identity

To describe who is using the space, an identity block or group, similar to the
one in usage record must be supported. As with usage record it must be possible
to specify both a local user name and global user identity. However the common
case is likely to be a VO or a VO group, so being able to describe this is
extremely important. Another use case is a local group at the site who owns the
data (not everyone uses grid). How exactly to describe the virtual organization
is not quite clear, but the following elements are needed as a base: VO name,
VO issuer (DN of the VO issuer, somewhat VOMS specific), VO group, and VO role.
There might be use cases for being able to have multiple VO blocks (though I
suspect that will be messy).

Resulting elements:
VO Name
VO Issuer
VO group
VO role
Global User Identity
Local User Name
Local User Group

All element are strings. The Global User Identity is to be considered global
unique. Furthermore the combination of VO issuer and VO name should be globally
unique.


5. Record Structure

Having identified the identity block and the site/storage structure, we tried
to find a good way to structure the individual records. We considered the
possibility of having a tree structure in the storage accounting record, with
the site, storage system, storage system partition, and storage type as
possible levels in the tree. The leaves would then each have an identity and
space usage block. This however would be quite complicated to construct and
parsed, so a simpler - more flat - structure was quickly preferred. Here each
record would maximum have a one site, storage system, storage system partition,
and storage type (but all still optional). If a system would need to describe
for multiple storage system / storage system partitions / storage type
several records would have to be generated. This format should still be easy
to aggregate together into site or storage system to get complete numbers. The
flat format is also much easier to explain which probably means that it should
be preferred. The two structure can describe exactly the same, so there is no
limitation by choosing the flat format.


6. Record Overview

Given the previous section, we can now present an overview of the elements in a
storage accounting record. We reuse the recordId and createTime elements from
the usage record standard (just the element names, not the namespaces).


StorageAccountingRecord

  recordId (considered globally unique)
  createTime (timestamp when the record was created)
  StartTime (datetime value)
  EndTime (datetime value)
  Site
  StorageSystem
  StorageSystemPartition
  StorageType
  StorageClass
  IdentityBlock
   VO Block
    VO Name
    VO Issuer
    VO Group
    VO Role
   GlobalUserIdentity
   LocalUserName
   LocalGroupName
  UsedSpace
  ReservedSpace
  UnallocatedSpace
  FileCount

The only enforced element is the recordId.


7. Name Issues

Someone brought up that SAR is not the most fortunate name. It sounds a bit
like SARS, and the phonetic sound is apparently close to a non-to-fortunate
Hungarian word.

We could consider SR (storage record) or SUR (storage usage record)
This really doesn't matter.


8. Sharing Elements with Usage Record Standard

Some of the elements are identical in both name and semantics to the ones in
usage record. We do not suggest to share the elements as such (same namespace),
as it would make the standard rely on the UR standard, and hence make it less
self-contained. The UR standard is only used in a few systems, and is likely to
be replaced with a new standard sometime. Furthermore the implementation gains
of sharing the names are very small, if they even exist.


We have definitely missed something in this, but we hope this can be a start
for the discussion in Brussels. If you see problems or issues with this record
please let us know.


     Best regards, Henrik Thostrup Jensen & Jon Kerr Nilsen