sabi notesNotes mostly about Linux and computer issuessabicohttp://www.sabi.co.uk/Copyright 2015 Peter G.http://www.sabi.co.uk/2016-08-18T10:10:00ZMaking systems more fragile2015-12-19T17:20:00Z2015-12-19T17:20:00Zhttp://www.sabi.co.uk/blog/151219The boot process of GNU/Linux based systems is
becoming every more fragile, with many steps (some of them pointless)
and support for ever greater complexity in the numberr of storage
layers to configure to mount a system filetree.Ever faster PCIe based flash SSDs2015-12-16T20:20:00Z2015-12-16T20:20:00Zhttp://www.sabi.co.uk/blog/151216A recent 512GB PCIe flash SSD can do 2.5GB/s
reading, 1.5GB/s writing, costs $350, and has a couple of
interesting specification details.Container based deployment has been around a long time2015-11-14T14:10:00Z2015-11-14T14:10:00Zhttp://www.sabi.co.uk/blog/151114Containers, in the sense of systems running a
single application, have been around for a long time. They are
suitable for the case where applications are frozen in place because
maintenance is too expensive or difficult, and therefore isolating
each application reduces trouble. Otherwise when developers are
going to be better at maintenance than whoever is running a system,
for example for mobile phones and other consumer devices.CERN's old large disk discussion and IOPS-per-TB2015-03-29T15:40:00Z2015-03-29T15:40:00Zhttp://www.sabi.co.uk/blog/150329Looking back at CERN's 2007 target for
concurrent read-write rates per TB, that is in effect
IOPS-per-TB.Perhaps Amazon slices large disks across to implement cold storage2015-03-28T11:20:00Z2015-03-28T11:20:00Zhttp://www.sabi.co.uk/blog/150328Amazon's Glacier cloud archival storage is
unusually cheap for Amazon, and it has very long access delays, and
one guess I find plausible is that it used the unused inner
cylinders of disks otherwise used for low latency frequently used
data.Anisotropy and the difference between performance and speed2015-10-23T08:40:00Z2015-10-23T08:40:00Zhttp://www.sabi.co.uk/blog/151023Speed, a single metric, is not performance,
which is an envelope of possible tradeoffs.Canonical's community involvement model seems unwise2015-09-18T21:30:00Z2015-09-18T21:30:00Zhttp://www.sabi.co.uk/blog/150918Volunteer programnmers like to develop
half-begun low-quality but cool-looking applications themselves, and
don't like to finish such applications and improve their quality
when written by others (or themselves). Therefore free software
development involves volunteer programmers writing demos for free,
and commericial vendors paying employees to finish them. Canonical
has amazingly chosen to do the opposite and then they hope that
volunteers will turn their demos into products for free.Still happy with Dell U2412M and Acer B326HUL2015-08-23T18:50:00Z2015-08-23T18:50:00Zhttp://www.sabi.co.uk/blog/150823My two most recently purchased monitors
still feel quite good.Fast MacBook Air builtin flash, Corsair USB3 stick2015-08-22T11:00:00Z2015-08-22T11:00:00Zhttp://www.sabi.co.uk/blog/150822The MacBoook Air builtin flash SSD can read and
write at 1.2GB/s and an inexpensive USB3 stick can read at 170MB/s
and write at 70MB/s.Small shell functions to set window and tab labels2015-04-06T11:00:00Z2015-04-06T11:00:00Zhttp://www.sabi.co.uk/blog/150406Some slightly updated shell functions to set the
title of some virtual terminal windows or tabs.PCIe flash SSDs and heat2015-04-18T09:35:00Z2015-04-18T09:35:00Zhttp://www.sabi.co.uk/blog/150418The CPU chip of compact M.2 format flash SSDs
can reach 110C during ordinary operation.Long running test of flash SSD endurance ends2015-04-06T18:20:00Z2015-04-06T18:20:00Zhttp://www.sabi.co.uk/blog/150406bIn a simple test of nearly continuous
writing all consumer grade flash SSDs tests lasted at least
700TBs and some 2400TB over 18 months.Two better IPC schemes2015-03-21T11:05:00Z2015-03-21T11:05:00Zhttp://www.sabi.co.uk/blog/150321Message passing and shared memory are well
known IPC schemes that don't require virtual memory. If virtual
memory is available two better alternatives exist: to move segments
from one address space to another so that data neither gets copied
nor shared, and to move threads (including their stacks) from one
address space to another.How many VMs per disk arm?2015-03-05T23:20:00Z2015-03-05T23:20:00Zhttp://www.sabi.co.uk/blog/150305Often consolidating many physical systems
onto a VM hosts consolidates storage too, resulting in a switch
from each physical system having dedicated access to a disk
arm to sharing it with other virtual machines. Unfortunately
sharing disk capacity is much easier and cheaper than sharing
IOPS.Very high density flash SSD box has 500TB in 1U2015-03-15T14:55:00Z2015-03-15T14:55:00Zhttp://www.sabi.co.uk/blog/150315A Silicon Valley company has introduced an
interesting "big" flash SSD storage system that packs over 500TB
in a 1U chassis. Quite useful for low power and space, and also
for high IOPS-per-TB, but perhaps not worth the company valuation,
even if that much power and storage density is not trivial.10TB drives with shingled tracks2015-03-14T16:10:00Z2015-03-14T16:10:00Zhttp://www.sabi.co.uk/blog/150314A 10Tb disk drive with shingled tracks has been
announced. Hopefully it will be cheap, as it will be used mostly as
a somewhat-random access tape cartridge, for archival. Since it
used shingled tracks it needs RMW, but then for archival it should
not be a significant issue.Some secondary properties of RAID142015-03-02T19:50:00Z2015-03-02T19:50:00Zhttp://www.sabi.co.uk/blog/150302RAID14 has some useful secondary properties
that mean it may be overall preferable to RAID15, even if plain RAID10
is still better.A script to print the status of a UMTS 3G connection2015-02-22T10:30:00Z2015-02-22T10:30:00Zhttp://www.sabi.co.uk/blog/150222Most UMTS 2G ("mobile broadband")
modems have a similar AT-style command language, and can
print periodic status lines. I have written some years ago
a small 'perl' script that does that and prints status in
a compact and readable form. it is not polished, but it is
quite useful for me.The interfaces that are not system calls2015-01-31T11:10:00Z2015-01-31T11:10:00Zhttp://www.sabi.co.uk/blog/150131There are several critical kernel
interfaces in Linux that are not implemented using system calls, in
part because the system call API is meant to be stable. This is
but a symptom that Linux is an agglomeration of specialized kernels
that share some common resource management infrastructure, and each
is designed differently with a different style of interface.Expecting magic when writing large files slowly2015-02-03T18:50:00Z2015-02-03T18:50:00Zhttp://www.sabi.co.uk/blog/150203Some users were so surprised when
writing large files slowly resulted in them being
fragmented. If the filesytem cannot anticipate how large a
file will be it cannot keep extension space reserved
indefinitly. Fortunately there are several way for user
programs to hint to the file system expected access patterns
and even the final size of a file, but very few programs use
them, unsurprisingly as more important storage practices
are not very commonly used.Setting KDE SC 4 network availability manually2015-01-31T10:35:00Z2014-10-25T12:10:00Zhttp://www.sabi.co.uk/blog/141025The KDE system status subsystem sometimes
gets wrong whether the system has network connectivity, but there
is a way to set that status manually.