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Add a project page for tracking work that needs to be done to support

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large filesystems and storage arrays.
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Scott Long 2004-09-12 18:33:10 +00:00
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# Summary of work needed to support large disks and arrays.
#
# $FreeBSD$
MAINTAINER= scottl
.if exists(../Makefile.conf)
.include "../Makefile.conf"
.endif
.if exists(../Makefile.inc)
.include "../Makefile.inc"
.endif
DOCS= index.sgml
DATA= style.css
.include "${WEB_PREFIX}/share/mk/web.site.mk"

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" [
<!ENTITY base CDATA "../..">
<!ENTITY date "$FreeBSD$">
<!ENTITY title "Large data storage in FreeBSD">
<!ENTITY % includes SYSTEM "../../includes.sgml"> %includes;
<!-- Status levels -->
<!ENTITY status.na "<font color=green>N/A</font>">
<!ENTITY status.done "<font color=green>Done</font>">
<!ENTITY status.wip "<font color=blue>In progress</font>">
<!ENTITY status.untested "<font color=yellow>Needs testing</font>">
<!ENTITY status.new "<font color=red>Not done</font>">
<!ENTITY status.unknown "<font color=red>Unknown</font>">
<!-- The list of contributors was moved to a separate file so that it can
be used by other documents in the FreeBSD web site. -->
<!ENTITY % developers SYSTEM "../../developers.sgml"> %developers;
]>
<html>
&header;
<h2>Contents</h2>
<ul>
<li><a href="#background">Purpose and background</a></li>
<li><a href="#testing">Testing large capacities</a></li>
<li><a href="#userland">Userland Tools Status</a></li>
<li><a href="#ifnet-status">Kernel Driver Status</a></li>
</ul>
<a name="background"></a>
<h2>Purpose and background</h2>
<h3>The UFS filesystem</h3>
<p>When the UFS filesystem was introduced to BSD in 1982, its use of 32
bit offsets and counters to address the storage was considered to be
ahead of its time. Since most fixed-disk storage devices use 512 byte
sectors, 32 bits allowed for 2 Terabytes of storage. That was an almost
un-imaginable quantity for the time. But now that 250 and 400 Gigabyte
disks are available at consumer prices, it's trivial to build a hardware
or software based storage array that can exceed 2TB for a few thousand
dollars.</p>
<p>The UFS2 filesystem was introduced in 2003 as a replacement to the
original UFS and provides 64 bit counters and offsets. This allows for
files and filesystems to grow to 2^73 bytes (2^64 * 512) in size and
hopefully be sufficient from quite a long time. UFS2 largely solved
the storage size limits imposed by the filesystem. Unfortunately, many
tools and storage mechanisms still use or assume 32 bit values, often
keeping FreeBSD limited to 2TB.</p>
<p>We need to ensure that FreeBSD supports large storage sizes and that
the benefits of UFS2 can actually be realized so that FreeBSD can remain
relevant in the enterprise world. This page describes known issues and
limits and provides a focus for further auditing, validation, and
fixing.</p>
<h3>Limits on disk partitioning</h3>
<p>The first limit that is encountered is in disk partitioning. For x86
and amd64 PC's, the FDISK MBR table is used by the BIOS to partition the
disk into logical extents and identify which partition ('slice' in FreeBSD
terms) to boot off of. The MBR is defined to use 32 bit disk offsets,
and since it's an industry standard and interoperability is required,
there is nothing that can be done to change this. As long as booting a
PC requires the MBR, the boot slice in FreeBSD is going to be limited to
2TB.</p>
<p>The GPT partitioning scheme was introduced with the ia64 architecture
as an MBR replacement. It provides 64 bit offsets and allows for an
arbitrary number of partitions. It also provides a compatibility mode
with MBR where it can generate an MBR-compatible structure on the disk
for use with systems that don't understand GPT. However, to get the
full benefits for boot storage, the BIOS and the FreeBSD loader must
understand it. For secondary storage, GPT can be used by any
architecture regardless of BIOS or boot support.</p>
<p>Many systems don't require an MBR or GPT, and even PCs don't require it
if booting and inter-operating with other OS's is not required. The next
limit that comes in, though, is with the BSD disklabel. This label
defines up to 8 partitions on a disk, MBR slice, or other storage extent
for filesystems and swap space. Unfortunately, the on-disk format of the
disk label again uses 32 bit quantities, so it is also limited to 2TB.
Fixing this would require creating a new format that is incompatible
with the old and would require an update to the FreeBSD boot loader.
This would complicate interoperability and the upgrade path. Also, if a
new format is going to be created, it should also address the 8 partition
limit that exists now. Given these requirements, it's tempting to just
adopt the GPT format instead for secondary storage partitioning.</p>
<a name="testing"></a>
<h2>Testing large capacities</h2>
<p>Even though large drives are cheap, it still isn't always feasible or
economical to test on real hardware. Swap-backed memory disks, via the
md(4) driver, can provide a good substitute for some of the testing.
Backing with swap means that only the pages that are dirtied by data
are actually allocated, so a multi-terabyte storage can be simulated
with a minimal of physical RAM+swap. Note that this is less true with
UFS1 since it will initialize all of the inode blocks during newfs,
which will dirty quite a bit of data. But for UFS2, swap-backed md
has the potential for working well. Unfortunately, the kernel md driver
has a number of 32-bit size limits of its own that need to be fixed.
Details are provided below.</p>
<p>It is still possible to avoid disklabels and MBRs for testing by
using newfs directly on the raw disk or md disk. Sysinstall can be
tested from a running system by just selecting Expert mode and just
performing the MBR and disklabel steps. Beware that sysinstall might
have other bugs that will wipe out your existing system, so care must
be taken here!</p>
<a name="userland"></a>
<h2>Userland Tool Status</h2>
<p>The following userland tools need auditing and testing for 64-bit
cleanliness:</p>
<table border=3>
<tr>
<th> Task </th>
<th> Responsible </th>
<th> Last updated </th>
<th> Status </th>
<th> Details </th>
</tr>
<tr>
<td>newfs_ffs</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&status.new;</td>
<td>A quick audit of newfs shows that the '-s' option uses atoi()
instead of strtoull() or equivalent. A more thorough audit is needed
to see if other integer limits exist.</td>
</tr>
<tr>
<td>df</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&status.new;</td>
<td>An audit is needed to make sure that all reported fields are
64-bit clean. There are reports with certain fields being incorrect
or negative with NFS volumes, which could either be an NFS or df
problem.</td>
</tr>
<tr>
<td>du</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&status.new;</td>
<td>An audit is needed to make sure that all reported fields are
64-bit clean.</td>
</tr>
<tr>
<td>growfs</td>
<td>&a.scottl;</td>
<td>12 Sept 2004</td>
<td>&status.wip;</td>
<td>Growfs has problems with expanding to new cylinder groups. It also
initializes UFS2 inode blocks instead of leaving them for lazy
initialization. It also needs a 64-bit audit.</td>
</tr>
<tr>
<td>sysinstall</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&status.new;</td>
<td>A full audit is needed. Reports exist of problems with >1TB
partitions.</td>
</tr>
<tr>
<td>fsck_ffs</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<td>&status.new;</td>
<td>A full audit is needed.</td>
</tr>
</table>
<a name="Kernel"></a>
<h2>Kernel Driver Status</h2>
<p>Many storage peripherals simply are not designed to handle >2TB
capacities. For those that are, an audit should be done to verify
that their drivers handle the sizes correctly and pass those sizes
correctly to the test of the kernel.</p>
<table border=3>
<tr>
<th> Task </th>
<th> Responsible </th>
<th> Last updated </th>
<th> Status </th>
<th> Details </th>
</tr>
<tr>
<td>md</td>
<td>&a.scottl;</td>
<td>12 Sept 2004</td>
<td>&status.wip;</td>
<td>A number of sizes and offsets are tracked using the 'unsigned'
data type, so it appears that it cannot comprehend sizes greater
than 2TB with a 512 byte sector size. For the swap-backed module,
the page counter is also stored as a 32-bit quantity which also
might be a limiting factor.</td>
</tr>
</table>
&footer;
</body>
</html>

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