Kernel Coverage at LWN
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XFS online filesystem scrubbing and repair
In a filesystem track session at the 2018 Linux Storage, Filesystem, and Memory-Management Summit (LSFMM), Darrick Wong talked about the online scrubbing and repair features he has been working on. His target has mostly been XFS, but he has concurrently been working on scrubbing for ext4. Part of what he wanted to discuss was the possibility of standardizing some of these interfaces across different filesystem types.
Filesystem scrubbing is typically an ongoing activity to try to find corrupted data by periodically reading the data on the disk. Online repair attempts to fix the problems found by using redundant information (or metadata that can be calculated from other information) stored elsewhere in the filesystem. As described in Wong's patch series, both scrubbing and repair are largely concerned with filesystem metadata, though scrubbing data extents (and repairing them if possible) is also supported. Wong said that XFS now has online scrubbing support, but does not quite have the online repair piece yet.
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Supporting multi-actuator drives
In a combined filesystem and storage session at the 2018 Linux Storage, Filesystem, and Memory-Management Summit (LSFMM), Tim Walker asked for help in designing the interface to some new storage hardware. He wanted some feedback on how a multi-actuator drive should present itself to the system. These drives have two (or, eventually, more) sets of read/write heads and other hardware that can all operate in parallel.
He noted that his employer, Seagate, had invested in a few different technologies, including host-aware shingled magnetic recording (SMR) devices, that did not pan out. Instead of repeating those missteps, Seagate wants to get early feedback before the interfaces are set in stone. He was not necessarily looking for immediate feedback in the session (though he got plenty), but wanted to introduce the topic before discussing it on the mailing lists. Basically, Seagate would like to ensure that what it does with these devices works well for its customers, who mostly use Linux.
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Using user-space tracepoints with BPF
Much has been written on LWN about dynamically instrumenting kernel code. These features are also available to user-space code with a special kind of probe known as a User Statically-Defined Tracing (USDT) probe. These probes provide a low-overhead way of instrumenting user-space code and provide a convenient way to debug applications running in production. In this final article of the BPF and BCC series we'll look at where USDT probes come from and how you can use them to understand the behavior of your own applications.
The origins of USDT probes can be found in Sun's DTrace utility. While DTrace can't claim to have invented static tracepoints (various implementations are described in the "related work" section of the original DTrace paper), it certainly made them much more popular. With the emergence of DTrace, many applications began adding USDT probes to important functions to aid with tracing and diagnosing run-time behavior. Given that, it's perhaps not surprising that these probes are usually enabled (as part of configuring the build) with the --enable-dtrace switch.
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