The London Stock Exchange experienced two halts to trading impacting securities that trade on matching engine Partition 3 on Tuesday 5 December. All constituents of the FTSE 100 and FTSE 250 indices and International Order Book securities continued to trade as normal throughout.

The cause of the halts was the degradation of processing performance of a disk array. The degradation was insufficient to cause a process fail-over to its mirror but instead triggered a protective request to suspend matching engine Partition 3.

The first halt, at 09:16:58, was triggered by a data management system which experienced a data processing backlog as a function of the disk array degradation and went into a protective state triggering the suspension of matching engine Partition 3. During the halt, this process was failed over to its mirror and the market restarted with order-driven securities recommencing in a re-opening auction at 09:55 and uncrossing at 10:15. Quote driven securities were recommenced by permitting quoting. SETSqx securities recommenced as per their normal schedule ahead of their next scheduled auction, uncrossing at 11:00.

The second halt, at 11:20:57, was triggered by another service on the same disk array that had not previously been experiencing performance issues, which triggered a separate protective matching engine Partition 3 suspension. Once all necessary processes were moved from the impacted hardware, the market was restarted with order driven securities recommencing in a re-opening auction at 12:20 and uncrossing at 12:40. Quote driven securities were recommenced by permitting quoting. SETSqx securities recommenced as per their normal schedule ahead of their next scheduled auction, uncrossing at 14:00.

Following each halt, affected order driven instruments transitioned into a re-opening auction but their Instrument Status was not updated via Group Ticker Plant (GTP) and therefore appeared to still be halted when they were in fact in the auction. These securities received an Instrument Status update following the re-opening auction uncrossings to correctly reflect their trading schedule. SETSqx securities received an Instrument Status update following their next scheduled auction uncrossing to correctly reflect their trading schedule. Quote driven securities did not receive an Instrument Status update following each resumption and so appeared as halted until end of trading but were available for quoting as normal. Trade reporting was available as normal throughout the halts.

As order driven securities on matching engine Partition 3 transitioned into the closing auction at 16:30 as scheduled, Instrument Status messages were not disseminated via GTP. The root cause of the GTP behaviour is being investigated.

Steps are being taken overnight to replace the impacted hardware. Systems are being prepared to open as normal on Wednesday 6 December.

The cause of the halts was entirely different to the software logic issue related to the Request For Quote (RFQ) functionality for Exchange Traded Products (ETPs) that impacted matching engine Partition 3 on 19 October 2023.

Source: LSE