iowait - When Storage Problems Wear a CPU CostumeWhen a disk stops responding, the CPU doesn't go idle — it goes into iowait, and many tools (SNMP included) roll that into a single "CPU load" number. So a storage failure shows up as a CPU alarm. This is how one such alert on a production ClickHouse node led down through iowait and a hung txg_sync to the real cause - a single EBS volume, no redundancy, that briefly lost access.Can SayinSeptember 15, 20266 min readclickhousezfszreplzpoolawsebs
The Null That Wouldn't Die - A ClickHouse ReplacingMergeTree Debugging StoryA real production debugging story — how a ReplacingMergeTree table with no version column lets the wrong row quietly win, and why "just delete the table" would have hidden the real bug.Can SayinSeptember 14, 202613 min readclickhousereplacingmergetreedebuggingdata-engineering
MongoDB Aggregation Pipeline - A Practical Guide From $match to $lookupA hands-on guide to the MongoDB aggregation pipeline. Master $match, $group, $sort, and $lookup by building a real reporting query one stage at a timeCan SayinSeptember 10, 202611 min readmongodbmongodb aggregationmongodb matchmongodb groupmongodb sortmongodb lookupmongodb unwind
The ESR Rule - How to Build the Perfect Compound Index in MongoDBYour index exists but queries are still slow? It's usually field order. Learn the ESR rule — Equality, Sort, Range — to build MongoDB compound indexes that actually work.Can SayinSeptember 9, 20267 min readmongodbperformancemongodb indexmongodb index esr
ClickHouse Disaster Recovery with ZFS Snapshots and zreplA practical guide to continuously replicating ClickHouse data off-box using ZFS snapshots and zrepl — why it works, how to set it up, how to restore, and the consistency caveats nobody mentions.Can SayinSeptember 8, 202612 min readclickhousezfszrepl
HAProxy vs CHproxy for ClickHouse: Which Load Balancer Should You Use?A practical guide to load balancing ClickHouse. When HAProxy wins, when chproxy wins, minimal setup for both, and the key parameters that actually matter.Can SayinSeptember 7, 20269 min readclickhousehaproxychproxy
